Peripheral blood (PB) from two children with so-called juvenile-type (Ph1 chromosome negative) "chronic granulocytic" leukemia (CGL) was found to contain cells capable of forming large numbers of exclusively monocytic colonies in vitro. These results are markedly different from those reported for classic adult-type (Ph1 chromosome positive) CGL where predominantly granulocytic colonies are produced in vitro from PB. Spontaneously dividing PB and bone marrow (BM) cells from one patient contained a translocation between chromosome 3 and a C-group chromosome; phytohemagglutinin-stimulated PB cells from this patient had normal chromosome morphology, however. Since the translocation was present in 141 of 142 karyotypes from the BM at a time when it contained many dividing erythroid and myeloid cells, the pathologic process is not exclusively confined to the monocyte line, but involves all hematopoietic cells. So-called juvenile CGL is neither a chronic nor a granulocytic leukemia; it is a panmyelopathy with monocyte predominance and should thus be classified as a variant of myelomonocytic leukemia.