We describe two patients with mevalonate kinase deficiency and prominent hematologic abnormalities and cholestatic liver disease. Patient R.B. was not anemic at birth, but developed petechiae and cutaneous extramedullary hematopoiesis, hepatosplenomegaly, leukocytosis, and recurrent febrile events without positive bacterial or viral cultures. Patient N.M. manifested minor anomalies, hepatosplenomegaly, anemia, thrombocytopenia, recurrent febrile crises, and facial rashes. Mevalonic aciduria was found by urinary organic acid analysis, and mevalonate kinase deficiency was documented in both. The clinical spectrum of normocytic hypoplastic anemia, leukocytosis, thrombocytopenia, and abnormal blood cell forms led to diagnoses of congenital infection, myelodysplastic syndromes, or chronic leukemia in these patients before recognition of mevalonate kinase deficiency. Mevalonate kinase deficiency represents a single-gene abnormality that may be associated with significant hematologic findings. Recognition of the variability of this disorder with some patients manifest-ing only mild neurologic findings, yet significant hepatosplenomegaly, normocytic anemia, thrombocytopenia, and leukocytosis is important for all specialists who need to be aware of this organic aciduria. Am. J.