Together with Rudolf Vrba he wrote up the story of his experiences in Slovak as Auschwitz, Tomb of Four Million People, a factual account of the Wetzler–Vrba report and of other witnesses.

Wetzler is known for the report that he and his fellow escapee, Rudolf Vrba, compiled about the inner workings of the Auschwitz camp–a ground plan of the camp, construction details of the gas chambers, crematoria and, most convincingly, a label from a canister of Zyklon B.

The publication of parts of the report in June 1944 is credited with helping to persuade the Hungarian regent, Miklós Horthy, to halt the deportation of that country's Jews to Auschwitz, which had been proceeding at a rate of 12,000 a day since May 1944.

On 26 June, Richard Lichtheim of the Jewish Agency in Geneva sent a telegram to England calling on the Allies to hold members of the Hungarian government personally responsible for the killings.

[7] Horthy resisted Hitler's threats, and Budapest's 200,000–260,000 Jews were temporarily spared from deportation, until the pro-Nazi Arrow Cross Party seized power in Hungary in a coup on 15 October 1944.