The pathogenic microbial flora of genital ulcers in 161 (80 men and 81 women) unselected patients was studied prospectively. In only one case was Treponema pallidum responsible whereas herpes simplex virus was considered to be the cause of 130 (80.8%) genital ulcers. H ducreyi was isolated from 46 (28.6%) patients, most commonly as a secondary pathogen in herpetic lesions. Two or more pathogens were isolated from the ulcers in 67 (41.6%) patients, and in 21 (13%) patients no pathogens were isolated. Our results indicate an urgent need for antiviral treatment to reduce the local reservoir of genital herpes, challenge traditional concepts about the prevalence of H ducreyi in Britain, and call for a reappraisal of its role in the causation of genital ulcers.