Irregular use of the pill compromises the effectiveness of this highly reliable method. The consistency of pill-taking has traditionally been estimated through women's own reports of their patterns of pill use. In this study, self-reported data on pill-taking were compared with data from an electronic device measuring compliance among 103 women attending university health services and publicly funded family planning clinics. In three months of pill use, the electronic and self-reported data agreed on the number of days when pills were missed only 45% of the time; the level of agreement dropped from 55% in the first month to 38% in the third month. In each month, the proportion of women reporting no missed pills was much higher than the proportion recorded electronically (53-59% compared with 19-33%), and the proportion missing at least three pills according to the electronic data was triple that derived from the women's reports (30-51% vs. 10-14%). In addition, the electronic data recorded substantially more episodes in which women missed pills on two or more consecutive days (88 vs. 30).