Epididymitis is a medical condition in which there is inflammation of the epididymis (a curved structure at the back of the testicle in which sperm matures and is stored). This condition may be mildly to very painful, and the scrotum (sac containing the testicles) may become red, warm and swollen. It may be acute (of sudden onset) or rarely chronic.

Epididymitis is the most frequent cause of sudden scrotal pain. In contrast with men who have testicular torsion, the cremaster reflex (elevation of the testicle in response to stroking the upper inner thigh) is not altered. If the diagnosis is not entirely clear from the patient's history and physical examination, a Doppler ultrasound scan can confirm increased flow of blood to the affected epididymis.

Infection is the most common cause. In sexually active men, Chlamydia trachomatis is the most frequent causative microbe, followed by E. coli and Neisseria gonorrhoeae. In children, it may follow an infection in another part of the body (for example, a viral illness), or there may be an associated urinary tract anomaly. Another cause is sterile reflux of urine through the ejaculatory ducts. Antibiotics may be needed to control a component of infection. Treatment otherwise comprises pain killers or anti-inflammatory drugs and bed rest if necessary, and symptom control by resting the scrotum in a supported position.