Summary

Background Sexual behaviour is a major determinant of sexual and reproductive health. We did a National Survey of Sexual Attitudes and Lifestyles (Natsal 2000) in 1999–2001 to provide population estimates of behaviour patterns and to compare them with estimates from 1990–91 (Natsal 1990). Methods We did a probability sample survey of men and women aged 16–44 years who were resident in Britain, using computer-assisted interviews. Results were compared with data from respondents in Natsal 1990. Findings We interviewed 11 161 respondents (4762 men, 6399 women). Patterns of heterosexual and homosexual partnership varied substantially by age, residence in Greater London, and marital status. In the past 5 years, mean numbers of heterosexual partners were 3·8 (SD 8·2) for men, and 2·4 (SD 4·6) for women; 2·6 % (95 % CI 2·2–3·1) of bot