Miklos Nyiszli was a Hungarian Jewish pathologist imprisoned in the Nazi concentration camps, principally Auschwitz. At Auschwitz, Josef Mengele used him as a pathologist; he would likely have been killed had he not accepted. His testimony and writing about Mengele and Auschwitz in general is widely accepted as authoritative, although the focus of many attacks from Holocaust denialists.
Nyiszli later gave a 1945 deposition against Mengele and wrote a 1960 book about his experiences, republished in 1993. Describing his first view of Auschwitz, he saw on arriving in 1943: "...an immense square chimney built of red bricks tapering towards the summit. I was especially struck by the enormous tongues of flame rising between the lightning rods....I tried to realize what hellish cooking would require such a tremendous fire....A faint wind brought the smoke towards me. My nose, then my throat, were filled with the nauseating odor of burning flesh and scorched hair."
Auschwitz and Mengele
Nyiszli attempted, as best he could, to maintain his medical ethics. His relationship with Mengele, though, was complex. At times, it seemed that they worked together as colleagues, although that was conflictual for him and led to some loss of integrity. The last line of his book is "I would begin practicing again, yes, but I swore that as long as I lived I would never lift a scalpel again."  The scalpel referred to the autopsies he had done for Mengele; he has not been accused of direct experimentation on living people.
Describing a relatively close moment, "A long afternoon in deep discussion with Dr. Mengele, trying to clear up a number of points [during which] I was no longer a humble ... prisoner, and I...defended and explained my point of view as though this was a medical conference of which I were a full-fledged member." He reflected "I know men, and it seemed to me that my firm attitude, my measured sentences and even my silences were qualities by which I had succeeded in making Dr. Mengele, before whom the SS themselves trembled, offer me a cigarette in the course of a particularly animated discussion, proving he forgot for a moment the circumstances of our relationship." 
Lifton points out there were discrepancies between Nyiszli's 1945 deposition, and his book.  Even basic courtesy was rare in the concentration camps, and the proffer of a cigarette, seemingly a minor event, was both unauthorized and far beyond the norms of SS behavior.
He characterized Mengele as "among malefactors and criminals, the most dangerous type is the 'criminal doctor', especially when he is armed with powers such as those granted to Dr. Mengele."
SelectionsObserving Mengele during Nazi selection, he called him "indefatigable." Elaborating, he wrote
Any person who entered the gates of the KZ was a candidate for death. He whose destiny had directed him into the left-hand line was transformed by the gas chamber into a corpse within the hour. Less fortunate was he whom adversity had singled out for the right-hand column. He was still a candidate for death, but with this distinction — that for three months, or as long as he could endure, he had to submit to all the atrocities the KZ had to offer 'til he dropped from utter exhaustion. 
The New York Review of Books called his book "the best brief account of the Auschwitz experience available."
It was attacked by the Holocaust revisionist, Charles Provan, at the 2000 conference of the revisionist Institute for Historical Review. Provan argues against a considerable amount of Holocaust history, but agrees the gas chambers were real. This is but one of many attacks by revisionists. Provan, however, did confirm Nyszli's education, visit to the U.S., and presence at Auschwitz, but argues that the book is effectively a novel. 
A 2002 movie, "The Dead Zone", used the book as one of its primary sources. 
- Miklos Nyiszli (1993), Auschwitz: a doctor's eyewitness account, Arcade, p. 23
- Nyiszli, p. 160
- Nyiszli, p. 137
- Robert Jay Lifton (1986), The Nazi Doctors: medical killing and the psychology of genocide, Basic Books, p. 370
- Nyiszli, p. 52
- Gerald Astor (1983), The Last Nazi: the life and times of Dr. Joseph Mengele, Donald H. Fine, ISBN 091765746, p. 62
- Gerald Posner and John Ware (1986), Mengele: the Complete Story, McGraw-Hill, ISBN 0070505985, p. 26
- Neal Ascherson (28 May 1987), "The Death Doctors", New York Review of Books
- Charles Posner (May-June 2000), "13th IHR Conference: A Resounding Success", Journal of Historical Review 19 (3): 2-11
- Charles Provan (January/February 2001), "New light on Dr. Miklos Nyiszli and his Auschwitz book", Institute for Historical Review
- Elbert Ventura (18 October 2002), "The Dead Zone", Popmatters