To investigate the diversity of morphology in adult T-cell leukaemia/lymphoma (ATL) and its possible association with the pathophysiology of ATL, we selected 36 acute cases and 14 chronic cases phenotypically confirmed to have >90% ATL cells in peripheral blood mononuclear cells. Prototype ATL cells were observed in all cases, although the percentage of all lymphoid cells varied considerably (48.9 +/- 23.8 in acute type, 29.6 +/- 18.9 in chronic type; P = 0.015). Chronic lymphocytic leukaemia (CLL)-like morphology with round nuclei was more frequent in chronic type than in acute type (52.0 +/- 24.9% v 16.6 +/- 13.1%; P < 0.0001). Unusual morphology (UM; lymphoblastic, vacuolated, granular pleomorphic or large cells) was more frequent in acute type than in chronic type (20.1 +/- 18.7% v 2.7 +/- 3.2%; P < 0.0001). Furthermore, there were significant negative and positive correlations of % CLL-like cells and % UM cells respectively, with serum LDH level, hypercalcaemia, performance status, and total number of involved lesions. Cases with aberrant immunophenotype (n = 6) or defective HTLV-1 integration (n = 22) showed lower % CLL-like cells and higher % UM cells than other cases, respectively. Cases with >50% CLL-like cells (n = 7; all chronic type) were younger (53.1 +/- 12.2 v 66.9 +/- 10.6 years; P = 0.038) and showed longer acute-crisis free survival (mean: 16.7 v 3.0 years; P = 0.012) than chronic cases with <50% CLL-like cells. These results suggest that diversity in genotype, phenotype, morphology and behaviour of ATL are closely associated, and that CLL-like morphology is a good prognostic factor for chronic type.