BACKGROUND: Sexually transmitted infection (STI) risk is determined both by partner numbers and partnership characteristics. Studies describing only recent partnership(s) overestimate long-term partnerships and underestimate the contribution of casual partnerships to STI transmission in populations. We describe all heterosexual partnerships in the past year in terms of partnership type, age and geographical mixing and how these characteristics relate to condom use. METHODS: Probability sample survey of 11 161 men and women aged 16-44 resident in Britain, 1999-2001. Computer-assisted self-interviews asked respondents about partner numbers and detailed questions about their three most recent partnerships. We weight these data to represent partnerships for which detailed questions were not asked to present estimates for the population of partnerships. RESULTS: Of 15 488 heterosexuals partnerships, 39.1% (95% CI 36.6-41.7%) of men's partnerships were 'not (yet) regular' vs 20.0% (95% CI 18.2-21.9%) of women's partnerships. While condoms were used at last sex in 37.1% (95% CI 35.0-39.3%) of men's and 28.8% (95% CI 27.1-30.6%) of women's partnerships, and for 55.3% (95% CI 52.6-58.0%) of first sex with new partners, these proportions declined with age. When partnerships involved an age difference of 5+ years [26.2% (95% CI 23.0-29.6%) of men's and 36.5% (95% CI 33.0-40.1%) of women's partnerships], condoms were less commonly used at first sex than when partners were closer in age [44.1% (95% CI 39.1-48.4%) vs 60.8% (95% CI 57.3-64.2%)]. Sex occurred within 24 h in 23.4% (95% CI 19.7-27.5%) of men's and 10.7% (95% CI 8.3-13.6%) of women's partnerships. CONCLUSIONS: A substantial minority of partnerships in the population is casual. The proportion of partnerships not protected by condoms is high, especially for partnerships involving larger age differences and people in their 30s and 40s. Condom use with new partners needs to be promoted among all age-groups.