AbstractBoth cytogenetic abnormalities and in vitro growth patterns of leukemic cells have prognostic significance in acute nonlymphocytic leukemia (ANLL). The present study was undertaken to analyze the interrelationship between these two factors and response to therapy. Blast cells from 43 patients with de novo ANLL, four patients with secondary ANLL, and two patients with ANLL following a preleukemic phase were studied using both in vitro culture in methylcellulose and quinacrine chromosome banding techniques. In 19 patients with de novo ANLL, minimal growth in culture (≤ 5 colonies without prominence of small clusters) was noted (Pattern I). Blast cells from the remaining 24 patients formed numerous abnormal clusters and colonies in a continuum distribution (Pattern II). Sixty‐three percent of Pattern I patients had completely normal karyotypes (NN). Only 29% of Pattern II patients were NN. No Pattern I patient had only abnormal karyotypes (AA), while 25% of Pattern II patients were AA (p<0.05). All six patients with secondary leukemia or leukemia following a preleukemic phase demonstrated both Pattern II growth and cytogenetic abnormalities. Fifty‐six percent of Pattern I patients with de novo ANLL responded to chemotherapy, 35% of Pattern II patients responded, and 0% of patients with secondary or post‐preleukemic ANLL responded. The correlation between the presence of cytogenetic abnormalities in leukemic cells and a pattern of excessive, abnormal growth in vitro coupled with the trend toward poor prognosis in these patients suggests that cytogenetically abnormal cells may have a proliferative advantage both in vitro and