A 5-month-old white girl having persistent oral candidiasis was brought to medical attention because of acute respiratory distress, pneumonia, and hypoxia that worsened despite supportive care and antibiotics. Bronchial lavage fluid yielded Pneumocystis carinii. The diagnosis of acquired immunodeficiency syndrome (AIDS) was suspected, although enzyme-linked immunosorbent assay (ELISA) and Western blot tests were both negative for human immunodeficiency virus (HIV) antibody. Immunologic evaluation included the following results: a low normal CD4/CD8 ratio 0.88, CD4 lymphocytes 493/microL, and elevated IgA 539 mg/dL and IgM 175 mg/dL with normal IgG 492 mg/dL. Lymphocyte stimulation study results were depressed. Lymphocytes sent for culture were subsequently positive for HIV. The mother was HIV antibody positive by enzyme-linked immunosorbent assay and Western blot but belonged to no high-risk group and was asymptomatic except for chronic diarrhea. The father was HIV antibody negative. The patient was treated with pentamidine and IV gamma-globulin with good clinical response and a rapid decrease of IgM and IgA toward normal values. Subsequent candidal pneumonia and candidal esophagitis were treated successfully with amphotericin B. The patient has received prophylactic IV gamma-globulin infusions for 6 months and remains HIV negative by enzyme-linked immunosorbent assay and Western blot. This case of pediatric AIDS highlights the need to consider HIV infection in the differential diagnosis of any child with physical findings or illnesses suggestive of AIDS-related complex or AIDS, even when HIV serologic findings are negative and parents belong to no high-risk group. Parental testing for HIV antibody is suggested in such cases.