SUMMARYThe causes of acute conjunctivitis in 140 consecutive patients attending an ophthalmic casualty department in London were investigated. Laboratory evidence of infection with Chlamydia trachomatis, adenovirus, or herpes simplex virus was found in 52 (37%) cases. In 70 cases (50%) no evidence of infection with these agents was found. In the remaining 18 cases (13%) laboratory results were inconclusive. The importance of these findings, the role of laboratory investigations in the differential diagnosis of acute conjunctivitis, and their cost-benefit are discussed.Acute conjunctivitis (AC) is a common reason for attendance of patients at ophthalmic casualty (outpatient) departments. Most ophthalmologists take little interest in AC because of difficulties in making aetiological diagnoses and the assumption that AC is a self-limiting and benign disease.