To assess the etiologic role of C. trachomatis and other micro-organisms in "idiopathic" epididymitis, 23 men underwent microbiologic studies, including cultures of epididymal aspirates in 16. Eleven of 13 men under the age of 35 years had C. trachomatis infection whereas eight of 10 over 35 had coliform urinary-tract infection. Cultures of epididymal aspirates yielded C. trachomatis alone in five of six men under 35, and coliform bacteria alone in five of 10 over 35. These results suggest that C. trachomatis is the major cause of "idiopathic" epididymitis, and coliform bacteria the major cause of epididymitis in older men. Expressible urethral discharge and inguinal pain were more common in the chlamydial cases, whereas concurrent genitourinary abnormality and scrotal edema and erythema occurred more commonly in the coliform cases. The morbidity attributable to C. trachomatis is as serious as that attributable to Neisseria gonorrhoeae.