33 cases of chronic granulocytic leukemia (CGL) were reassessed to determine if, by strict morphologic criteria. Philadelphia chromosome (Ph1)-negative CGL exists as a diagnostic entity and if Ph1-positive CGL could be distinguished from Ph1-negative CGL. Cases were reassessed using published criteria and, of 11 Ph1-negative cases, only 4 could be reclassified as myelodysplastic syndromes or undifferentiated chronic myeloproliferative disorder. Of the morphologic parameters evaluated, peripheral blood basophilia and bicytopenia proved to be good discriminators between Ph1-positive and Ph1-negative cases. As a group, Ph1-negative cases were more heterogeneous and tended to have lower hemoglobin, WBC, platelet count and absolute eosinophilia. Chromosomal abnormalities other than Ph1 were seen only in the Ph1-positive cases. Based on these findings, we conclude that Ph1-negative CGL constitutes a heterogeneous group, a subgroup of which is morphologically identical with the Ph1-positive CGL. The parameters that best discriminate between Ph1-positive and Ph1-negative cases are peripheral blood absolute basophilia and bicytopenia.