We report severe aplastic anemia of neonatal onset diagnosed in six girls between 1985 and 1995 in a single center. Initial blood cell counts (mean age 3.8 days old, 1 to 15 days) showed thrombocytopenia (six of six), anemia (four of six), and neutropenia (two of six). Neutrophil counts gradually decreased below 0.5 x 10(9)/L, and severe aplastic anemia occurred in three patients by 3 months of age and in all patients by 1 year of age. Lymphocyte number and functions were normal. In all children bone marrow biopsy showed hypocellularity for age and absence of fibrosis, blasts, lymphocytic infiltrates, and cytologic abnormalities. Blood and medullary cytogenetic studies were normal. A search for known constitutional, viral, or toxic causes was negative. Immunosuppressive therapy failed to restore hematopoiesis (three of six). Five children received a bone marrow transplantation at an average age of 9 months (range 2.7 to 29 months). One child is alive and well after a human leukocyte antigen-identical bone marrow transplantation, whereas the other four died. Both congenital onset and the high rate of familial involvement suggest that this condition may be inherited.