The prevalence of sexually transmitted infection was studied in 40 men presenting with acute epididymo-orchitis in Leeds. Chlamydia trachomatis infection was identified in 13 of 29 men (45%) aged under 35 years. Neisseria gonorrhoeae was isolated from four of these 13 men with chlamydial urethritis. C trachomatis was isolated from the urethra of only one of 11 men (9%) aged over 35. Appreciable bacteriuria was found in six of these 11 men (55%). Noting details of sexual history and screening for sexually acquired pathogens is advocated in younger men with acute epidiymo-orchitis.