Expression patterns of interleukin-6 (IL-6), IL-6 receptor (IL-6R), and gp130 genes in 39 patients with acute myeloid leukemia (AML), in 23 patients with acute lymphoblastic leukemia (ALL), and in 7 patients with acute mixed lineage leukemia (AMLL) were studied by quantitative reverse transcriptase-polymerase chain reaction. Significant levels of IL-6 were expressed in 8 (21%) of 39 AML patients and in 2 (29%) of 7 AMLL patients, whereas in ALL, the expression of IL-6 was almost negligible. IL-6R was expressed in all patients with AML and AMLL, whereas only half of ALL patients expressed low levels of IL-6R as compared with those with AML and AMLL. However, gp130 was ubiquitously expressed in all the leukemia patients, and there was no significant difference in gp130 expression among AML, ALL, and AMLL. Significant correlation was observed between the expression of IL-6R and gp130 in AML. When tested for in vitro response to IL-6, the leukemic cells from 3 of 7 AML, none of 3 ALL, and both of 2 AMLL patients significantly responded to IL-6, showing the correlation between the expression levels of IL-6R and gp130 and the responsiveness of leukemic cells to IL-6. These results showed that quantitation of IL-6R and gp130 expression by reverse transcriptase-polymerase chain reaction is useful for the rapid prediction of the responsiveness of leukemic cells to IL- 6, especially in cases of administration of IL-6.