Twenty-one patients with CD30 (Ki-1) positive lymphoma were studied from a group of 91 patients with adult T-cell leukaemia/lymphoma. The patients were grouped into three types: diffuse CD30 positive anaplastic large cell lymphoma in 11 patients (group 1); pleomorphic type lymphoma with diffuse CD30 expression in five patients (group 2); and pleomorphic type lymphoma with positive CD30 expression in large cells but negative in medium-sized and small cells in five patients (group 3). The patients with diffuse CD30 positive lymphomas (groups 1, 2) frequently presented with extranodal tumours (68.8%) and lymph node enlargement greater than 2 cm in diameter (50%), and rarely with leukaemic changes, bone marrow involvement and hypercalcaemia (one case of each). Patients in group 3 rarely had extranodal tumours, but had frequent leukaemic changes. Expression of intercellular adhesion molecule (ICAM-1; CD54) by the lymphoma cells in 13 patients (81.3%) with diffuse CD30 positive lymphomas, was significantly higher than that in 33 patients (9.1%) with CD30 negative adult T-cell leukaemia/ lymphomas. No positive reaction for epithelial membrane antigen (EMA) was found in the lymphoma cells of CD30 positive cases. The overall survival in patients with diffuse CD30 positive lymphomas was better than that of CD30 negative adult T-cell leukaemia/lymphoma patients, but showed no significant difference. These findings suggest that diffuse CD30 positive adult T-cell leukaemia/lymphoma has unusual clinical and immunohistological findings. It is also speculated that local tumour formation and leukaemic changes in such diffuse CD30 positive cases are influenced by CD54 (ICAM-1) expression by the lymphoma cells.