To produce concentrations of zidovudine (AZT) in plasma and cerebrospinal fluid that would provide constant inhibition of the replication of human immunodeficiency virus (HIV), we gave AZT by continuous intravenous infusion to 21 children ranging in age from 14 months to 12 years who had acquired HIV infection through transfusions or perinatally. All patients were symptomatic before AZT treatment (Class P2 of the Centers for Disease Control); 13 (62 percent) had evidence of neurodevelopmental abnormalities. The mean CD4/CD8 ratio was 0.18; 11 patients had CD4 counts below 0.2 x 10(9) per liter. We administered AZT at four dose levels: 0.5, 0.9, 1.4, and 1.8 mg per kilogram of body weight per hour. The plasma drug concentrations achieved at the respective dose levels were 1.9 +/- 0.3, 2.8 +/- 1.4, 3.1 +/- 1.1, and 4.5 +/- 1.0 microM. The steady-state cerebrospinal fluid:plasma ratio was 0.24 +/- 0.07. The only evidence of toxicity was bone marrow suppression. Transfusion was required in 14 patients because of low levels of hemoglobin (5 mmol per liter [less than 8 g per deciliter]). Dose-limiting neutropenia (less than 0.5 x 10(9) polymorphonuclear leukocytes per cubic millimeter) occurred in most patients who received doses of 1.4 mg per kilogram per hour or more. Improvement in neurodevelopmental abnormalities occurred in all 13 children who had presented with encephalopathy before treatment. Serial measurements of IQ before therapy and after three and six months of continuous therapy with AZT showed that IQ scores, including those for verbal and performance IQ, rose in these 13 patients and in 5 other children who had no detectable evidence of encephalopathy before treatment. Most patients also had increased appetite and weight, decreased lymphadenopathy and hepatosplenomegaly, decreased immunoglobulin levels, and increased numbers of CD4 cells. In some patients the improvement in the features of encephalopathy occurred despite the absence of immunologic improvement. We conclude that AZT is beneficial in children with symptomatic HIV infection, especially those with encephalopathy (which may be subclinical), and that the optimal continuous intravenous dose of AZT in children is between 0.9 and 1.4 mg per kilogram per hour.