The National Cancer Institute, in response to widespread public interest, undertook a retrospective analysis of Laetrile treatment. Only cases thought to have shown objective benefit from Laetrile were solicited by mail request to 385,000 physicians and 70,000 other health professionals and by direct contact with pro-Laetrile groups. Although it is estimated that at least 70,000 Americans have used Laetrile, only 93 cases were submitted for evaluation. Twenty-six of these Laetrile cases had to be eliminated because of insufficient documentation, and an equal number of conventionally treated cases selected from the institute's files were added to the records to be analyzed. A panel of 12 oncologists, who had no knowledge of the actual treatments given, was then asked to evaluate the results of 160 courses of treatment (68 Laetrile, 68 chemotherapy, 24 "no treatment") in the abstracted records from 93 patients. The panel judged six Laetrile courses to have produced a response (two complete and four partial). These results allow no definite conclusions supporting the anti-cancer activity of Laetrile. The National Cancer Institute will use the data in deciding if further study is needed.