From 1980 to 1990, 174 peripheral T cell lymphomas were studied morphologically and immunophenotypically with a panel of monoclonal antibodies which were reactive with T cell differentiation antigens in cryostat sections and/or cell suspensions. Histologically, 57% of the lymphomas were categorized into low-grade tumors according to the updated Kiel classification, while 41% were high-grade tumors. By immunologic studies, 50% of the lymphomas were of helper/inducer (CD4) phenotype, 6% were of cytotoxic/suppressor (CD8) phenotype, 3% expressed both CD4 and CD8, 3% lacked both CD4 and CD8, and 36% were phenotypically undetermined because of an admixture of a fairly even number of CD4 and CD8-positive cells. The phenotypically undetermined cases were more frequently noted in the low-grade groups than in the high-grade group, and the latter often showed a loss of pan-T antigens, although there was no definite correlation between the histologic category and the immunophenotype. CD25, which is strongly manifested in anti-HTLV-1 antibody-positive cases, was negative or only weakly expressed in anti-HTLV-1 antibody-negative cases. Anaplastic large cell lymphomas (LC-Ana) strongly expressed CD30, which was also detectable in only large blast-like cells in the low-grade tumors. Seventy-one per cent of the lymphomas expressed Ia antigens. In this series, the clinical data were available on 154 patients. For individual markers, the expression of CD30 and HLA-DR were associated with a longer actuarial survival (P < 0.01 and P < 0.05 by the generalized Wilcoxon test). The absence of CD25 or the presence of CD3 on tumor cells correlated with a relatively favorable prognosis, but not significantly. The detection of CD4 and CD8 had relatively little prognostic value. In the cases excluding LC-Ana, a significant difference was also recognized between the groups with and without CD25, CD30 and HLA-DR (P < 0.05 by the generalized Wilcoxon test). These results suggest that the immunophenotypic analysis of peripheral T cell lymphoma provided its use as an adjunct to a histopathologic diagnosis and was related to prognostic prediction.