Results of trials of various birth control methods and contraceptive products may provide misleading data and engender unrealistic expectations regarding efficacy. An analysis of published efficacy-trials reveals numerous fallacies in their design, performance, and reporting. Consequently, family planning clinicians find it virtually impossible to make valid comparisons among the methods or products. This article reviews the definitions and measures that have been used to assess contraceptive efficacy, describes and illustrates some of the flaws that confound interpretation and comparison of studies, and presents a set of recommendations for future studies. A summary table providing comparative failure rates for all methods of contraception is included.