Computer-stored drug-dispensing data for 143,574 outpatients were used to identify users of various medicinal drugs during the 4-year period beginning in July 1969. These patients were followed through 1976 for the development of cancer. This report presents the results of screening analyses for 95 commonly used drugs and drug groups in relation to 56 primary cancer sites and combinations of sites. Statistically significant positive or negative associations with at least one site were found for 53 drugs. Some of these associations were undoubtedly due to chance sampling variation; some confirmed previous reports; some suggested interesting new hypotheses concerning possible drug effects or other etiologic or preventive factors. Further study is required before conclusions as to causality can be reached. Although the duration of follow-up in this study is relatively short so far, the absence of associations for many of the drugs studied provides some provisional assurance of their lack of carcinogenic effects.