Abstract
The Royal College of General Practitioners' Oral Contraception Study, started in 1968/9, is reviewed. The limitations of any cohort study and the particular characteristics of this study mean that only large effects on common diseases are likely to be detected. Effects on conditions that might be subject to reporting bias, and on those, like cancers, which may only appear after a long time lag, are particularly unlikely to be estimated with any accuracy. The study does show a clear connection between oral contraceptive use and cardiovascular disease. It also suggests a range of conditions, including depression and some cancers, whose incidence might be increased by oral contraceptive use, and a few whose incidence might be decreased.