OBJECTIVE: To assess the contraceptive effectiveness of condoms versus oral contraceptive pills and estimate the reproductive consequences of a major shift from pill to condom use. METHODS: Secondary analysis was performed on nationally representative cross-sectional surveys of women in 16 developing countries. FINDINGS: In the 16 countries, the median per cent of married couples currently using condoms was 2%, compared with 13% for the pill. Condom users reported a higher 12-month failure and higher method-related discontinuation rates than pill users (9% and 44% vs 6% and 30%, respectively). Condom users were more likely to report subsequent abortion following failure (21% vs 14%), and also more likely to switch rapidly to another method (76% vs 58%). The reproductive consequences, in terms of abortion and unwanted births, of a hypothetical reversal of the relative prevalence of condom and pill were estimated to be minor. The main reason for this unexpected result is that the majority of abortions and unwanted births arise from non-use of any contraceptive method. CONCLUSION: A massive shift from the more effective oral contraceptive pills to the less effective condom would not jeopardize policy goals of reducing abortions and unwanted births. However, such a shift would potentially have an added benefit of preventing human immunodeficiency virus (HIV) infections, especially in countries with generalized HIV epidemics.