Twelve neonates in 3 cohorts received blood transfusions from two donors who were infected with human immunodeficiency virus (HIV). All 12 infants developed laboratory and/or clinical evidence of HIV infection, usually in the first year of life. Ten of 12 infants had serum antibody to HIV when tested between 9 and 42 months of age. The two seronegative infants were severely hypogammaglobulinemic when they were tested. Nine infants developed a variety of illnesses attributable to HIV infection, but only 2 fulfilled criteria for the diagnosis of acquired immunodeficiency syndrome. In follow-up ranging from 2 1/2 to 4 years 5 patients (42%) have died. Four patients had HIV-associated illnesses but recovered and now have few if any symptoms attributable to HIV infection. Three children have never had signs or symptoms attributable to HIV. Immunologic abnormalities were present in all patients; the most consistent finding was a decrease in the proportion of T helper cells. Three patients had severe panhypogammaglobulinemia. The hypogammaglobulinemic infants had significantly lower numbers and percentages of T helper cells compared to the remaining patients (P less than 0.01). We conclude that exposure to HIV via transfusion in the neonatal period results in an extremely high rate of infection with substantial mortality and morbidity, but clinical recovery occurs in some patients. Also hypogammaglobulinemia may be more common in infants with HIV infection than previously appreciated.