Apart from sexual abstinence and monogamous relationships between uninfected partners, the use of condoms is currently the only effective means available for preventing sexual transmission of human immunodeficiency virus (HIV) infection. Worldwide data on people's knowledge about condoms and their use have not previously been studied in the context of prevention of HIV transmission. This paper presents survey data taken among women of reproductive age in the developing countries. The results show a wide range in the women's knowledge of condoms, which was poorest in sub-Saharan African countries, including those that are considered to have high HIV seroprevalence. Use of condoms was lowest in sub-Saharan Africa and very low in some other countries with a high prevalence of HIV.