Involvement of the central nervous system by human immunodeficiency virus is an important cause of morbidity and mortality. We have undertaken a longitudinal study of asymptomatic individuals found to be human immunodeficiency virus seropositive to identify and characterize cerebrospinal fluid abnormalities early in the disease process. Our findings in 25 individuals have been notable for a frequent incidence of cerebrospinal fluid abnormalities. Pleocytosis or elevated cerebrospinal fluid protein was found in 12 (48%) of 15 patients studied. Oligoclonal banding was present in 6 (26%) of 23 patients. Human immunodeficiency virus was isolated by culture in 4 asymptomatic patients. The cerebrospinal fluid abnormalities we observed indicate an active process occurring in the central nervous system, even in early human immunodeficiency virus infection in asymptomatic patients. Serial observation of these patients for development of neuropsychiatric findings may provide answers to the significance of cerebrospinal fluid abnormalities identified in these patients.