An unusual case of human immunodeficiency virus type 1 (HIV-1) infection in a child was studied. The child, identified as HIV-1 infected at 5 years of age, lived with his parents and a 3-year-old sister. HIV-1 infection was excluded in the mother and sister, but confirmed in the father, who was unaware of his infection and was in good health, apart from an atopic dermatitis on the face and limbs. A portion of the HIV-1 proviral envelope gene was amplified from the father's and child's peripheral blood cells, and the amplified products were cloned and sequenced. Phylogenetic analysis disclosed that the father's and child's viral sequences clustered together, and were clearly distinct from the sequence sets obtained from six epidemiologically unlinked mother-child HIV-1-infected pairs included in the analysis. HIV-1 variability was lower in the child's sequence set than in the father's, and the variability between father's and child's sequences was significantly lower than that found between epidemiologically unlinked cases (p < 0.001). An uncommon APGR motif on the tip of the V3 domain was found in both the father's and child's viral clones. These data, together with the epidemiological investigations, strongly suggest that the child acquired the infection from his father, possibly by exposure to bleeding skin lesions.