Leukemic cells of bone marrow and peripheral blood showing clusters of distinctive, large, uniform, rounded intracytoplasmic inclusions were found in a patient who had acute lymphoblastic leukemia. These membrane-bound inclusions ranged from 0.8 to 1.4 microgram in diameter and were clearly demonstrated on routinely stained preparations. Electron microscopy disclosed transitions between well-developed mitochondria and the electron-dense structures. The functional significance of these presumably altered mitochondria is uncertain. Whorled lamellar internal structures, in occasional fingerprintlike arrangements, may represent duplication of mitochondrial cristae and, functionally, may reflect increased metabolic rate or otherwise abnormal metabolism. Cytochemical studies demonstrated the presence of both periodic acid-Schiff positivity and nonspecific esterase activity in these inclusions, but no peroxidase or acid phosphatase was found. These findings support the lymphoid nature of the disorder. Immunologic surface membrane markers are consistent with this case being, as with the majority of acute lymphoblastic leukemias, of non-B, non-T cell type. Cytogenetic studies on cultured leukemic cells from bone marrow and peripheral blood showed a consistently abnormal karyotype associated with the presence of an XXY chromosomal constitution. Since no other tissues (e.g., dermal fibroblasts) were examined, the diagnosis of Klinefelter's syndrome cannot be confirmed. A causal relationship between the coexistence of the abnormal karyotype and the acute leukemia was not proven but was suspected.