Objective To provide basic information on pre- and extramarital sex in the general population and other factors related to HIV transmission. Design Cross-sectional household survey in 18 countries of the developing world, mainly in Africa and Asia. Respondents Representative samples of 1300–6995 individuals aged 15–49 years, interviewed in 1989–1993. Methods Face-to-face interviewing. Results We observed a huge variability between study sites, with the proportion of men reporting sexual contact outside regular partnerships in the last year ranging from 4 to 47%. Contacts with sex workers range from 1 to 25%. Women were much less likely to report non-marital sex than men. Conclusions This first cross-cultural attempt to examine aspects of sexual lifestyles suggests that broad generalizations about multiple-partner sexual networking in particular regions are misleading. Gender, marital status, age and a few other demographic correlates were disclosed as powerful determinants of sexual behaviour, although the strength of associations varied greatly between specific locations. Condom use was very low in most study sites.