Rudolf Vrba, who as a young man escaped from Auschwitz and provided the first eyewitness evidence not only of the magnitude of the tragedy unfolding at the death camp but also of the exact mechanics of Nazi mass extermination, died on March 27 at a hospital in Vancouver, British Columbia. He was 81.
His wife, Robin, said he died of cancer.
Dr. Vrba went on to become a distinguished medical researcher in Israel, England, the United States and Canada, writing dozens of papers.
But his greatest importance is as an author of a much different paper -- one with diagrams of gas chambers and crematories. With remarkable specificity gained from camp jobs that gave him unusual access to various corners of Auschwitz, including the gas chambers, Dr. Vrba told the unknown truth about it.
The report became known as the Auschwitz Protocol. When part of it were released in the summer of 1944, the United States government endorsed it as true. Neither Dr. Vrba's name -- he was born Walter Rosenberg -- nor that of his fellow escapee, Alfred Wetzler, was given, in order to protect their safety.
The names of two other escapees and a Polish Army major whose information was added to the final protocol also went unidentified. Many history books still omit the names, although the document itself is central to many discussions of the Holocaust. It was used as evidence at the Nuremberg trials.
Dr. Vrba's wife said his name, virtually unpronounceable in English, is generally mispronounced as VER-ba. But he made it known by telling his story, most notably in his 1963 autobiography, ''Escape from Auschwitz: I Cannot Forgive.'' His influence grew even more after he appeared in Claude Lanzmann's 1985 documentary, ''Shoah.''
''The strength of the Final Solution was its secrecy, its impossibility,'' he said in an interview in 2005 with The Ottawa Citizen. ''I escaped to break that belief that it was not possible. And to stop more killings.''
As the Holocaust enveloped European Jews and other groups vilified by the Nazis, news of the outrage seeped only gradually to the outside world. By early 1941, however, the British had learned about massacres, and later that year, Jan Karski, a leader of the Polish underground, informed President Franklin D. Roosevelt of the unfolding horror.
On Dec. 17, 1942, the Allies issued a statement saying Jews were being taken to the camp and killed.
But the specifics of what was happening at Birkenau, the part of Auschwitz devoted to extermination, began to come to general attention only in January 1944, when a report prepared by the underground there was smuggled out and reached officials in Washington and London. No action was taken, however.
Then, on April 4, an Allied spy plane over Poland happened to photograph Auschwitz while documenting construction of a synthetic-fuels plant. The next day, Siegfried Lederer escaped to warn Czech Jews.
On April 7, Mr. Vrba and Mr. Wetzler, who died in 1988, escaped. On April 24, they reached Zilina, in northern Slovakia, where they worked with Jewish leaders on their report. The two men each provided details with the other not present. Factual assertions were checked against records whenever possible.
The 32-page report was sent to the British and United States governments, the Vatican and the International Red Cross. Most important, it went to the leadership of Hungary's Jews, next on Hitler's list.
It had been the construction of a new rail spur to the gas chambers that prompted Mr. Vrba and Mr. Wetzler to risk their lives to try to warn Hungarian Jews, the last major intact Jewish community in Europe. They had heard Nazis talking about ''Hungarian sausage'' coming.
But Hungarian Jewish leaders did not issue a warning, a failure that has been long debated. It has been suggested that the leaders feared jeopardizing an ultimately unsuccessful deal they were then negotiating with Adolph Eichmann to save at least some Jews. There was also concern that there was too little time for effective action.
Soon, it was too late by any measure. On June 6, two more Auschwitz inmates, Arnost Rosin and Czeslaw Mordowicz, arrived in Zilina. They reported that trainloads of Hungarian Jews were being massacred.
''Already 200,000 of these I had tried to save, those whom I thought, indeed, I had saved, were already dead,'' Dr. Vrba wrote. That total would more than double.
Still, the escapees' alarms saved some Jews, at least 100,000 by most estimates. Allied pressure, especially threats to hold Hungary's leadership responsible, prompted Admiral Miklos Horthy, Hungary's regent, to stop deportations on July 9, 1944.
Mr. Vrba was born Walter Rosenberg in Topolcany, Czechoslovakia, on Sept. 11, 1924. Rudolf Vrba was the nom de guerre he adopted after joining the Czechoslovakian resistance. He later made the change legal.
The young Walter Rosenberg was barred from school at 15 because he was a Jew. He worked as a laborer until 1942, when he was arrested and deported, first to the Maidanek concentration camp and then to Auschwitz. His escape was harrowing: he hid under a woodpile while guard dogs sniffed just inches away.
After World War II, he earned his doctorate and did postdoctoral work in Prague. After his various posts as a medical researcher, he became a professor of pharmacology at the University of British Columbia from 1976 until the early 1990's.
In addition to his wife, he is survived by his daughter, Zuza Vrbova Jackson of Cambridge, England, and two grandchildren.
Dr. Vrba said that he had devoted 95 percent of his time to science and 5 percent to the Holocaust. In both, he pushed beyond facts toward larger interpretations.
He told The Jerusalem Post in 1998, for example, that he could understand why some people doubted the true dimensions of the Holocaust. There was nothing in their experience remotely comparable, he said.
Photo: Rudolf Vrba, seen here in a scene from the 1985 film ''Shoah.'' (Photo by Mario Lanzmann/''Shoah'')