Adenoviruses were isolated from the urethral swabs of 129 male patients in an STD clinic. After exclusion of patients with Chlamydia trachomatis or Neisseria gonorrhoea infections, 85 of the remaining 120 patients had urethritis, compared with 28 men with urethritis detected in a control group which was closely matched for age, sex, and date of specimen collection. This statistically significant difference suggests that genital adenovirus infection may be a cause of urethritis in some male patients.