The pattern of any future major heterosexual epidemic of acquired immunodeficiency syndrome (AIDS) will depend partly on sexual behaviour and condom use among heterosexuals. This survey was designed to provide information on patterns of sexual behaviour in New Zealand. A national sample aged 18 to 54 was selected using a random method and telephone interviews were administered to 2361 people, using a questionnaire based on the protocol developed by the Global Program on AIDS of the World Health Organization. The reported mean lifetime number of partners increased with age up to 25 to 29 years for women and 30 to 34 years for men, and declined at older ages. Fifteen or more lifetime partners were reported by 17 per cent of men and 4 per cent of women. Multiple partnerships in the previous 12 months were commonest in those aged 20 to 24. In this age group, 32 per cent of men and 20 per cent of women reported two or more partners. Recent condom use for contraception was reported by 23 per cent of men and 19 per cent of women. Use was highest amongst those aged 18 to 24, and decreased sharply with age. The true proportion of the population with many sexual partners may be higher than reported. These data will be useful in modelling approaches to estimate the likelihood of future heterosexual spread of AIDS. The data on lifetime numbers of partners suggest that sexual decisions depend not just on age and sex but also on the era, and thus on changing social values about sexual behaviour.