The gene for the thrombopoietin receptor, c-mpl, has been shown to be overexpressed at the mRNA level in acute myeloid leukemia (AML) and myelodysplastic syndrome. A recent study reported c-mpl mRNA overexpression in 60% of a small sample of AML patients, and this overexpression correlates with shorter complete remission but not with karyotype group. We quantified c-mpl protein expression in 107 cases of AML and 24 normal bone marrow and 12 normal peripheral blood samples by using Western blot analysis and radioimmunoassay (RIA). Western blot analysis revealed no detectable level of c-mpl protein in the normal samples, whereas trace amounts were detected by RIA. c-mpl protein expression was increased (> or = twice normal) in 65% of the AML cases. c-mpl protein expression was correlated with cytogenetic groups (P = 0.0009, Kruskal-Wallis test in rejecting the hypothesis that c-mpl expression was the same in different groups). Specifically, patients with favorable cytogenetic groups (t(8;21), inv16, and t(15;17)) had lower c-mpl protein expression (median 1.7 times normal), whereas patients with unfavorable abnormalities (+8, -5 or -7, and del(11)(q23)) and normal cytogenetics had high expression (3.1 and 2.85 times normal, respectively). The findings were the same when only the 61 untreated AML patients were considered. No statistically significant correlation between c-mpl expression and age or antecedent hematologic disorder was found. These results suggest that c-mpl protein overexpression in AML may play a role in the aggressiveness of this disease.