Unilateral swelling of the epididymis is a common clinical illness seen in sexually active men. It is usually quite painful, with fever and acute unilateral swelling of the testicle that is sometimes confused with testicular torsion. In the preantibiotic era, approximately 10 to 15% of untreated gonococcal infections resulted in epididymitis. In developed coun-tries, the two most common causes of epididymitis are N. gonorrhoeae and C. trachomatis, especially in younger men. In men older than 35 and in homosexual men, Enterobacteri-aceae and coagulase-negative staphylococci may also cause the disease, probably from reflux of infected urine into the epididymis. Treatment depends ondemonstration of the etiologic agent in urethral specimens or epididymal aspirates (see treatment of urethritis for additional considerations).