This article describes a method for assessing whether the blindness of a double-blind study is maintained and, if not, whether failure to maintain blindness could have invalidated the results of the study. The benefit of using the method is illustrated in a study of the effect of nicotine gum on the tobacco withdrawal syndrome. In that study, many subjects were able to identify which drug they received; however, a drug effect was present among subjects who correctly identified their drug, among subjects who incorrectly identified their drug, and among subjects who could not tell which drug they received. Thus, failure to maintain blindness could not have invalidated the results of the study. The authors believe double-blind studies should routinely assess blindness, and they recommend their method as a simple and easy way to assess blindness.