Severe thrombocytopenia was the initial manifestation of human immunodeficiency virus infections in 5 infants under 15 months of age. In only 2/5 cases maternal seropositivity was previously known while in the other 3 it was discovered after onset of the infants' thrombocytopenia. One infant died, due to intracranial hemorrhage, and the other 4 developed to disease stage IV. Human immunodeficiency virus infection must be considered in the diagnosis of thrombocytopenia.