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use for each and every act of intercourse.4 The use of condoms is recommended for individuals who have multiple partners, who have a primary partner who is infected, or who have a partner whose serostatus is unknown,5 although the absolute amount of protection they provide has not been accurately established. The effectiveness of the condom as a contraceptive provides insight into its usefulness as a barrier device capable of preventing HIV transmission. Defined as the proportionate reduction in pregnancies caused by use of a contraceptive method, effectiveness is estimated as one minus the ratio of two failure rates. The failure rate ratio is calculated by dividing the pregnancy rate associated with use of a contraceptive method by the rate related to no method use for a given time period.6 The likelihood of becoming pregnant during the first year of condom use ranges from 2.6% to 15.8%.7 The likelihood of pregnancy in a population not practicing contraception is estimated from groups such as the Hutterites, and is often assumed to be 85%.8 These probabilities can be transformed into rates,* providing an estimate of condom effectiveness for preventing pregnancy of 90.7% to 98.6%. When the research described in this article was conducted, Karen R. Davis was research assistant in the Department of Preventive Medicine and Community Health, the University of Texas Medical Branch, Galveston, TX. Susan C. Weller is professor in the Department of Preventive Medicine and Community Health, the University of Texas Medical Branch, Galveston. The authors gratefully acknowledge James Trussell for noting the need to incorporate the length of follow-up time in the analysis, George Papaevangelou for clarifying the condom usage categories in the Roumelioutou-Karayannis A et al. study, Judah Rosenblatt for assistance with the Poisson-based confidence intervals and Mark Vanlandingham for discussion. The Effectiveness of Condoms in Reducing Heterosexual Transmission of HIV