Serial samples from human immunodeficiency virus-infected infants in the first year of life were analyzed by quantitative human immunodeficiency virus polymerase chain reaction assays. Very high, persistent levels of plasma RNA and proviral DNA were detected throughout the study period, suggesting the absence of an effective immune response. Most patients had normal CD4 lymphocyte counts and were symptom free for the first 3 to 6 months despite high levels of viral replication. These findings support the evaluation of early intervention (before symptoms develop) and efforts to establish the predictive value of these assays.