174 peripheral T-cell lymphomas, classified according to the updated Kiel classification (Suchi), were immunophenotypically studied with a panel of monoclonal antibodies reactive with T-cell differentiation antigens in cryostat section and/or cell suspension. 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 almost equal number of CD4- and CD8-positive cells. The phenotypically undetermined cases were more frequently noted in low grade lymphomas than in high grade tumors, and the latter often showed the loss of pan-T antigens, although there was no definite correlation between histologic category and immunophenotype. CD25, strongly detectable in anti-HTLV-1 antibody-positive cases, was negative or weakly expressed in anti-HTLV-1 antibody-negative cases. Anaplastic large cell lymphomas (LC-Ana) strongly expressed CD30, which was sometimes detectable in only blast-like cells of low grade tumors. 71% of the lymphomas expressed Ia antigens. In this series, the clinical data were available for 154 patients. For individual markers, the expression of CD30 and/or HLA-DR was associated with a longer actuarial survival (p<0.05 by the generalized Wilcoxon test), but, in the population excluding LC-Ana, the significant difference was undetectable between the groups with and without CD30. Further, the absence of CD25 or presence of CD3 and/or CD7 on tumor cells correlated with a relatively favorable prognosis, but not significantly. The detection of CD4 and/or CD8 had relatively little prognostic value.