To examine the possibility that interleukin-9 (IL-9) may be involved in oncogenesis and the proliferation of adult T-cell leukemia (ATL) cells, we examined the expression of IL-9 mRNA and growth response to IL-9 in five human T-lymphotropic virus type-I (HTLV-I) infected T-cell lines and in primary leukemia cells in peripheral blood from eight patients with ATL (four acute ATL and four chronic). Four out of five cell lines expressed IL-9 mRNA not correlated with Tax expression. Primary ATL cells from all patients also expressed IL-9 mRNA not correlated with the clinical forms. Recombinant IL-9 showed growth enhancing activity in only one out of five cell lines and one out of eight patients' primary leukemic cells. These results suggest the infrequent involvement of IL-9 in the proliferation of ATL cells, both primary tumor cells and HTLV-I infected T-cell lines.