OBJECTIVE--To investigate patterns of sexual behaviour in men and women with genital ulcer disease (GUD) and their relevance to HIV-1 transmission. METHODS--A sexual behaviour questionnaire was administered by the same interviewer to all participants who were also entered into a study of the microbial aetiology of GUD. SETTING--City Health Sexually Transmitted Diseases Clinic, King Edward VIII Hospital, Durban, South Africa. PARTICIPANTS--100 Zulu men and 100 Zulu women. RESULTS--36 (%) of men and 36 (%) of women had continued with sexual intercourse despite GUD. Patients with donovanosis and secondary syphilis were more likely than those with other causes of GUD to have intercourse despite ulcers. During swab collection bleeding was observed from ulcers in 59 women and 26 men. Prostitutes were not identified and were rarely named as source contacts. Men had more sexual partners (190) than women (122) during the previous three months. Condom use was minimal. Men who migrated between urban and rural areas appeared to have the most sexual partners. Urban women had more partners than women from rural areas. CONCLUSIONS--Men and women with GUD are practising riskful sexual behaviour and could benefit from behaviour modification programmes. In this community men who travel between urban and rural areas and who present late with GUD that bleeds easily are probably the most important high-frequency HIV transmitter core group. A significant potential risk of blood to blood contact during sexual intercourse exists in patients with GUD.