Monthly hemochrome parameters were obtained during the first 2 years of age in 22 children with perinatal human immunodeficiency virus type-1 (HIV-1) infection and in 58 exposed seroreverted children. Timing and predictive value of hemochrome modifications were investigated. Exclusion criteria were hemoglobinopathies and zidovudine (AZT) treatment in pregnancy. When AZT treatment was undertaken children were eliminated from the study. From the second month of life red blood cell (RBC) counts, hemoglobin (Hb) concentrations, and hematocrit values were significantly lower in infected than in uninfected children. RBC counts progressively diverged in infected and uninfected children, and mean values in the former group never reached 4.10(12)/L. No difference was observed in Hb content ratios and RBC size parameters. At 2 months RBC counts, Hb concentrations, and hematocrit values below reference values were associated with a 15.8 (95% confidence limits [CL]: 5.5-48.8) relative risk of being infected. In infected infants aged 5 months a decrease in these parameters was associated with an 11.2 (95% CL: 1.6-77.8) relative risk of developing eventual severe clinical outcome. Low RBC counts, Hb concentrations, and hematocrit values may be included among predictive criteria in infants of HIV-1 infected mothers.