Women diagnosed during the period 1943-1990 and reported to the Danish Cancer Registry with invasive squamous-cell carcinomas of the uterine cervix, vulva, vagina or anus, together with those having pre-cancerous lesions (CIN III or carcinoma in situ) of the uterine cervix diagnosed in the period 1958-1990, were followed for the occurrence of subsequent lung cancer over 762,000 person-years. Overall, these patients developed 2 to 2 1/2 times more lung cancers than women in the general Danish population. Women in whom cervical cancer was diagnosed recently, and before the age of 45 years, had a 4.6 times elevated risk of developing lung cancer, while young women with vulvar or vaginal cancer were at a 4.0-fold elevated risk. Similarly, women in whom anal cancer was diagnosed before the age of 60 years were at a 3.5-fold increased risk of developing lung cancer. The present study supports the hypothesis that smoking is involved in the aetiology of ano-genital malignancies. The particularly high risk of developing subsequent lung cancers seen in women who were pre-menopausal (< 45 years) at the time of the ano-genital cancer diagnosis suggests that the effect of smoking in ano-genital carcinogenesis might be partly mediated through alterations in oestrogen metabolism. Alternatively, patients who developed their initial ano-genital cancer at a young age might harbour some genetic susceptibility which could explain their excess lung-cancer risk.