Material from 100 consecutive men who presented with penile ulcerations was tested for a variety of microorganisms. Fifty-one patients had microorganisms identified that were considered primary pathogens. The laboratory data were compared with the clinical diagnoses and the diagnostic accuracy and index of suspicion calculated. The diagnostic accuracy for lesions due to Treponema pallidum was 77.8%, Herpesvirus hominis 62.9% and Haemophilis ducreyi 33.3%, whereas the indexes of suspicion were 88.2%, 67.7% and 300% respecitively. These data show that the clinical diagnosis of genital ulcers is oftern inaccurate and that definitive diagnosis requires laboratory tests.