Four cases of acute nonlymphocytic leukemia with primitive basophilic differentiation are presented. In all four cases, study revealed Philadelphia chromosome negativity, and in none were there clinical findings of chronic granulocytic leukemia. In each case, the leukemic blasts contained granules that failed to stain for peroxidase content but stained positively with toluidine blue. The former result could have led to the misclassification of the cases as lymphoid leukemias. Three of the four patients had physical findings that may have been due to circulating histamine excess. The histochemical and clinical features of these cases suggest that certain examples of leukemia with basophilic differentiation represent a distinctive variant of acute nonlymphocytic leukemia.