A rare variant of acute promyelocytic leukemia (APL) is associated with basophilic differentiation. Such a patient presented with basophilia, headaches, and diffuse engorgement of superficial blood vessels, attributable to hyperhistaminemia. Karyotype analysis showed a clonal rearrangement of chromosome 12p13 in addition to the t(15;17). During treatment with all-trans-retinoic acid (TRA), the absolute basophil count rose steadily during the first week, then declined. By one month, the basophilia resolved, an abrupt rise occurred in both the platelet and absolute neutrophil count, and the bone core biopsy showed complete maturation of all cell lines. Abnormalities of chromosome 12p13 in acute myelogenous leukemia have been associated with basophilia. Since every cell in our patient with t(12p13;?) also had the t(15;17), we speculate that the basophilia was due to clonal evolution with acquisition of the t(12p13;?). In two out of five other reported cases, abnormalities of chromosomes known to be associated with basophilia were present in addition to t(15;17). It is possible that the basophilia in this variant is reactive; however, since TRA induces differentiation of leukemic promyelocytes into mature neutrophils, we speculate that the leukemic promyelocytes in our patient differentiated into basophils. Future studies employing either fluorescent in situ hybridization or polymerase chain reaction using a probe to the breakpoint on t(15;17) may establish whether or not the basophils derive from the leukemic clone.