PURPOSE: To elucidate the role and clinical spectrum of congenital lymphocytic choriomeningitis virus infection as a cause of chorioretinopathy, congenital hydrocephalus, and macrocephaly or microcephaly in the United States. METHODS: We performed complete ophthalmologic surveys of all residents at Misericordia, a home for the severely mentally retarded in Chicago, and prospectively evaluated all patients with chorioretinitis or chorioretinal scars during a 36-month period at Children’s Memorial Hospital, also located in Chicago. Sera for patients demonstrating chorioretinal scars (a sign of intrauterine infection) were tested for Toxoplasma gondii, rubella virus, cytomegalovirus, and herpes simplex virus and lymphocytic choriomeningitis virus antibodies. RESULTS: Four of 95 patients examined at the home had chorioretinal scars, and two of these patients had normal T. gondii, rubella virus, cytomegalovirus, and herpes simplex virus titers and dramatically elevated titers for lymphocytic choriomeningitis virus. Three of 14 cases of chorioretinitis at the hospital had normal T. gondii, rubella virus, cytomegalovirus, and herpes sim-plex virus titers and elevated lymphocytic choriomeningitis virus antibody titers. (A fourth case, diagnosed in 1996, was reported 2 years ago.) CONCLUSIONS: Lymphocytic choriomeningitis virus was responsible for visual loss in two of four children secondary to chorioretinitis in a population of severely retarded children. The six new cases of lymphocytic choriomeningitis virus chorioretinitis identified in these two populations over the last 3 years, compared with the total number ever reported in the United States (10 cases), suggests that lymphocytic choriomeningitis virus may be a more common cause of congenital chorioretinitis than previously believed. Because its consequences for visual and psychomotor development are devastating, we conclude that the workup for congenital chorioretinitis should include lymphocytic choriomeningitis virus serology, especially if T. gondii, rubella virus, cytomegalovirus, and herpes simplex virus titers are negative.