OBJECTIVE: To ascertain the contribution of contraceptive failure to unintended births and fetal loss in developing countries. METHODS: Nationally representative survey data from married women in 19 developing countries were analyzed. All surveys contained retrospective monthly calendars of contraceptive use and pregnancies for a 5-year period preceding each survey. Information on the intendedness of live births, ascertained earlier in the interview, were linked to the calendar data. Single-decrement life table analysis was applied to episodes of use to estimate failure probabilities. The reproductive consequences of failure were established by simple tabulation. Logistic regression was used to explore the determinants of fetal loss. RESULTS: Reported contraceptive failure rates were similar to those derived from studies conducted mainly in the United States. About three fourths of pregnancies resulting from contraceptive failure were carried to term, and all but 16% of those were classified by the mother as unwanted or mistimed. Just over one tenth ended in fetal loss, either induced or spontaneous. Analysis of determinants of fetal loss suggested that a large proportion were induced. The median contribution of failure to all unintended births for all 19 surveys was about 15%, and the contribution to fetal loss was 12%. CONCLUSION: The contribution of contraceptive failure in developing countries is much lower than the estimate of 50% in the United States. Despite the substantial increases in contraceptive practice that have occurred in Asia, Latin America, the Middle East, and to a lesser extent, Africa, the level of use is still below the 75% mark achieved in most industrialized countries. Nonuse of contraception remains the dominant direct cause of unintended births, and family planning promotion should remain a public health priority.