Peripheral T-cell lymphoma (PTCL) accounts for 15-20% of non-Hodgkin's lymphoma in the Western World. Clinical, histopathologic, phenotypic and genotypic data were received from 33 cases of PTCL referred to our institution. The median age order was 50 years, 78% were males, and 18% had a history of a preceding disorder of the lymphoid system. 60% had stage 4 at diagnosis and B symptoms were also present in 60%. The most frequent sites of extranodal involvement were bone marrow (54%), liver (45%) and skin (33%). Twenty-eight of 33 cases were histologically classified according to the Working Formulation (most in the diffuse mixed and large-cell subgroups) and the Kiel updated system. Phenotypic and genotypic studies of malignant cells showed a considerable heterogeneity with respect to the expression of either T-cell receptor (TCR) alpha beta and gamma delta and pan-T differentiation molecules. Of the studied cases 63% expressed TCR-alpha beta. All five patients with PTCL of the TCR-gamma delta subtype had a peculiar extra-nodal presentation. The vast majority of cases expressed an abnormal T-cell phenotype with respect to the expression of pan-T antigens, including the lack of expression of the TCR-associated CD3 molecule in 2 cases. Rearrangements of the TCR beta and/or delta-chain genes showed clonality in 21 of the 23 studied cases. Twenty-five patients were treated with a multiagent chemotherapy regimen with curative intent and the remainder received a less intensive palliative regimen. Only 9 patients achieved CR (8 of whom had received an anthracycline-containing regimen) and the 4-year survival rate was 25%.(ABSTRACT TRUNCATED AT 250 WORDS)