With the use of the concentration and volumetric bone marrow aspiration technics in the differential diagnosis of idiopathic acquired aplastic anemia from aleukemic lymphatic leukemia in children, it is possible to establish an early reliable diagnosis. In this clinic specific therapy with the indicated agents has not had to await an indefinite period of observation pending the development of unmistakable clinical and peripheral blood signs nor the more difficult procedure of surgical biopsy. Bone marrow aspirations performed in four cases of aplastic anemia and studied by these technics have shown normal or increased ease and rate of aspiration, normal or increased fat, and absent myeloid-erythroid layer. Five cases of aleukemic leukemia presented as typical of numerous others studied revealed marked difficulty in the aspirations, absent fat and a variable myeloid-erythroid layer. Study of histologic material obtained with the use of the preceding technics revealed the marrow from all cases of aplastic anemia to be fatty and hypoplastic in contrast to the marrow from a case of aleukemic leukemia, which was hyperplastic and without fat tissue. These later observations correlate well with the observed differences in volumetric data.