HTLV-I infection of peripheral mature T cells appears to induce the expression of cellular genes including those of some cytokines and their receptors. We examined the expression of interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-2, IL-3, IL-4 and granulocyte/macrophage colony-stimulating factor (GM-CSF) at the mRNA level in fresh leukemic cells from 20 adult T cell leukemia patients to see whether there is any association between cytokine expression and HTLV-I expression and between their expression and clinical manifestations such as hypercalcemia or neutrophilia. IL-1 alpha, IL-1 beta and IL-3 expression was observed in 3, 7 and 1 of 20 cases examined, respectively. However, there seemed to be no association between IL-1 expression and clinical manifestations. IL-2, IL-4 and GM-CSF mRNA expression was not detected. HTLV-I viral RNA expression was detected only in one case in which IL-3 mRNA was expressed in both peripheral blood and lymph node cells and a relatively high proportion of leukemic cells expressed IL-2 receptor (p55, Tac). Thus, in the present study we could not find any correlation between cytokine expression and HTLV-I expression in peripheral blood fresh leukemic cells except in one unusual case.