Two children who presented initially with a lymphoid malignancy were noted to develop recurrences with myeloid features late in the course of their disease. In both cases, evidence of lymphoid differentiation was present in the myelogenous cells that were Ph1 chromosome negative. The first patient had acute myelogenous leukemia and developed a recurrence with morphologic features of acute myelogenous leukemia. Terminal transferase was present in the myelogenous blasts. The second patient initially had a diffuse lymphoblastic non-Hodgkin lymphoma. During the course of her illness she developed a myeloproliferative disorder characterized by basophilic meningitis, splenomegaly, and hypereosinophilia. Lymphocyte T-cell (E-rosette) markers were present on the eosinophils. These observations lend further support to the hypothesis of varying lymphoid and myeloid differentiation in certain cases of leukemia.