Adult T-cell leukemia/lymphoma (ATLL) is a malignancy involving peripheral blood, lymph nodes, skin and other organs. Human T-cell leukemia virus type I (HTLV-I) is etiologically associated with ATLL but cannot be detected by conventional methods in fresh samples of peripheral blood and skin taken from ATLL. The aim of this study was to investigate the feasibility of an in situ hybridization technique for detection of HTLV-I mRNA in atypical lymphoid cells of peripheral blood and skin lesions of patients with ATLL. We detected variable amounts of HTLV-I tax mRNA in the nuclei and cytoplasm of these cells in fresh peripheral blood samples and skin lesions from ATLL patients, and also in asymptomatic HTLV-I infected donors to a lesser extent. Out of 10 patients with ATLL, 7 showed strong positive in situ hybridization whereas the other 3 were only weakly positive. However, in the last 3 cases, the reaction became strongly positive after cells had been cultured for 24 hr. Furthermore, all 3 asymptomatic HTLV-I-infected donors exhibited a weakly positive response in their apparently mature lymphoid cells.