Objectives-To examine the effect of patient-defined non-regular heterosexual relationships on the incidence of sexually transmitted diseases and other genital infections in women and the role of condom use in the prevention of their spread. Design-A cross-sectional study of sexual behaviour reported by a standardised self-administered questionnaire in new patients who presented for screening and diagnosis. Setting-A genitourinary medicine clinic in West London. Subjects-938 consecutive newly attending women who completed a sexual behaviour questionnaire in 1992. Main outcome measures-Variables relating to socio-demographic status, sexual behaviour, condom use, sexually transmitted diseases and other genital infections stratified by the reporting of non-regular partners. Results-We found that women who reported non-regular sexual partners were more likely to be single (p = 0.0001), white (p < 0.0001), have had coitarche before 17 years of age (p = 0.003) and many more sexual partners both in the last year and in their life-time (p < 0.0001) and were more likely to practise fellatio (p < 0.0001), anal penetration (p = 0.004) and to be smokers (p < 0.0001). Paradoxically, the incidence of sexually transmitted diseases and other genital infections was no higher in this group than in the group of women who did not have non-regular partners. Increasing condom use with regular partners correlated with decreasing incidence of gonorrhoea (p < 0.001), chlamydial infection (p < 0.01) and trichomoniasis (p < 0.02), but increasing condom use with non-regular partners did not show this trend. Conclusions-Regular heterosexual partners play the major role in transmission of bacterial sexually transmitted diseases to women. This is significantly influenced by use of condoms.