To examine the relationship between the expression of human T-cell leukaemia virus type (HTLV-I) mRNA and associated antigens and clinicopathological features, we studied 31 lymph nodes of patients with adult T-cell leukaemia/lymphoma (ATLL) and related diseases, using in situ hybridization and immunohistochemistry. We classified the patients into four types on the basis of their clinicopathological features (HTLV-I associated lymphadenitis, incipient ATLL, ATLL with complete HTLV-I provirus, and ATLL with defective HTLV-I provirus. The expression of HTLV-I mRNA was detected in all 3 patients with incipient ATLL, in 5 of 10 patients with defective-provirus ATLL, in 5 of 11 patients with complete-provirus ATLL, and 3 of 7 with HTLV-I associated lymphadenitis, but the amounts were very small; approximately 1 in 10000-200000 lymph node cells express the viral genomes. This suggests that expression of viral genomes may not be important for immortalization, but it is important that to note the capacity for HTLV-I infection is preserved in each group of non-neoplastic and neoplastic states. HTLV-I mRNA was detected only in lymphocytes and/or lymphoma cells, but the HTLV-I associated antigens (env, gag and pX) were found in histiocytes and endothelial cells, as well as in lymphocytes and/or lymphoma cells. Anti-interleukin 2 receptor (IL-2R) antibody reacted with the giant cells of incipient ATLL and with the transformed lymphocytes and immunoblast-like cells of the HTLV-I-associated lymphadenitis but not with the lymphocytes in the background. Of the typical ATLL, IL-2R was found in both lymphoma cells and giant cells. IL-2 was rarely detected.