Twenty-eight patients with recent onset of nonvesicular ulcerative genital lesions were studied prospectively to define the microbial etiology. Causative agents identified included herpes simplex virus (HSV), Chlamydia trachomatis, Haemophilus ducreyi, and Treponema pallidum. Three women and 25 men (seven of whom were homosexual) were studied. Ten patients did not have an etiology identified. Nine patients had syphilis, six had genital herpes, one had chancroid, one had granuloma inguinale, and one had a human bite injury. Six of seven homosexual men had syphilis, as compared with one of eight heterosexual men (P less than .002). Among heterosexual men, five had atypical HSV infections, three with extensive balanoposthitis and two with chancroidal ulcers. Of the clinical characteristics examined, induration of the ulcer base and tenderness on palpation were predictive of etiology, whereas numbers of ulcers and regional lymphadenopathy were not. H. ducreyi was not a common cause of genital ulcers in Winnipeg, and it was not found as a commensal or superinfecting organism in ulcers caused by other agents.