The husbands of married women with cervical dysplasia, carcinoma-in-situ, or invasive carcinoma of the cervix who, in an earlier study, had claimed to have had no sexual partner other than their husband, were interviewed to determine whether the risk of cervical epithelial abnormalities in the women could be related to their husband's sexual background. Of 322 women previously studied, 57 were eligible for this study and the husbands of 31 of them were interviewed together with the husbands of a control group, matched for age and age at first intercourse. The number of sexual partners reported by the husband was found to be a significant risk factor, with a relative risk of 7.8 for 15 or more partners outside marriage. The relative risk for women who smoked was 7.0, and this was independent of any of the sexual risk factors. The findings for subgroups of women with invasive carcinoma and with dysplasia or carcinoma-in-situ were similar. The data strongly support the view that an infectious agent is involved in the aetiology of cancer of the cervix and suggest that smoking may have an independent carcinogenic action.