Penile lesions observed in United States Armed Forces personnel in Japan were often difficult to diagnose. Darkfield examinations for syphilis were seldom positive, and reports on the occurrence of Hemophilus ducreyi varied with the laboratory. To examine the problem, a study was made on a small, but well-controlled, group of patients. Emphasis was placed on thoroughness of laboratory examinations, with major consideration given to isolation and identification of microbial agents known to be responsible for penile ulceration. The results of this study are presented here. Study Groups Patients.— Thirty-five patients with penile lesions were referred to this laboratory from a nearby military installation. Histories were taken and physical examinations performed before specimens were collected for the laboratory. The patients examined were young (average age 23 years), unmarried (100%), noncircumcised (91%), venereal disease repeaters (71%), who drank before exposure to prostitutes (94%), and did not use prophylaxis (86%). Normal (Control) Subjects.—