To examine the relationship between human T-lymphotrophic virus type I (HTLV-I) proviral DNA and its expression in the lymph nodes, HTLV-I DNA and tax/rex mRDA were directly amplified by polymerase chain reaction in situ hybridization (PCR/ISH), and reverse transcription (RT)-PCR/ISH [RT-PCR/ISH]. We studied 24 lymph nodes from patients with adult T-cell leukemia/lymphoma (ATLL), incipient ATLL (I-ATLL), and HTLV-I associated lymphadenitis dermatopathic type (HAL-D) and enlarged paracortical type (HAL-EP). In ATLL, 40-60% of the nucleated cells were positive for for HTLV-I proviral DNA by PCR/ISH, while in I-ATLL and HAL, respectively 5-20% and less than 1-5% of cells were positive. The number of mRNA expressing cells was smaller than that of the proviral DNA-positive cells. The mRNA-expressing cells varied in number among the ATLL and I-ATLL cases, while they were only rarely observed in HAL-D and HAL-EP. These results show that HTLV-I infection and activation might increase with malignant transformation of the target T helper cells.