Patients with acute promyelocytic leukemia (APL) show other chromosome aberrations in addition to t(15;17) but their influence on the clinical outcome is still unclear. We have cytogeneticaly analyzed 43 APL patients with t(15;17)(q22;q21), treated with all-trans-retinoic acid (ATRA) according to the recommendations of the European APL 91 Group. Additional chromosome aberrations were observed in 14/43 patients (33%) studied at initial diagnosis. These patients were designed as 'complex' karyotype group and were compared to patients with t(15;17) asa sole cytogenetic abnormality ('simple' karyotype group). The 'complex' group had significantly lower platelet count and fibrinogen level and fewer cases without significant DIC at diagnosis than the 'simple' group. Comparison of 'simple' and 'complex' groups showed significant difference in complete remission rate (76% vs 35.7%, P = 0.0148) and early death rate (24% vs 64.3%, P = 0.0141). Survival analysis showed that the presence of additional chromosome abnormalities and significant DIC had an adverse effects on prognosis (P = 0.036 and P = 0.041, respectively), independent on other prognostic factors. These data indicate more aggressive biological nature of leukemic cells in patients with additional chromosome aberrations. Supplementary therapeutic strategies may be required for this subgroup of APL patients.