In medical experiments on human beings, every patient must sign an “informed consent” form acknowledging the risks, and researchers are required to keep track of those statements.

But the doctors who conducted a controversial, widely publicized lung cancer study involving more than 50,000 patients at numerous hospitals were unable to locate 90 percent of the consent forms, according to a confidential review provided to The New York Times.

The finding casts further doubt on a clinical trial that made headlines in 2006 when it concluded that fully 80 percent of lung cancer deaths could be prevented through wide use of CT scans.

That trial, led by Dr. Claudia I. Henschke at Weill Cornell Medical College in New York City, drew sharp criticism from skeptics of cancer screening; the criticism intensified when The Times reported in March 2008 that the research was being financed in part by $3.6 million in grants from the parent company of the Liggett Group, a cigarette maker.