In an 18 month period, 198 men presented with a painful, swollen, and tender epididymis or testicular or scrotal pain. Fifty were excluded from analysis because of prior antibiotic treatment or a history of the disease, or both. Of the remaining patients, epididymitis was not diagnosed in 108, though 23 (21%) of them had urethritis. Thus 40 men were seen who had acute unilateral epididymitis. Of the 27 less than 35 years old, 13 (48%) had a urethral chlamydial infection and two others a gonococcal infection. Sexually transmitted micro-organisms were not confined, however, to the younger age group, though only two (15%) of 13 men who were 35 years or older had a urethral chlamydial infection. Most, that is 29 (73%), of the patients with acute epididymitis also had urethritis when first seen. Urethral micro-organisms were found most often in 13 men who had severe epididymitis, chlamydial infection occurring in eight (62%) of the patients in this category.