Peripheral T-cell lymphoma (PTCL) consists of a diverse group of post-thymic tumors bearing a mature T-cell phenotype and, excluding mycosis fungoides, comprises approximately 10-20% of the non-Hodgkin's lymphomas in the United States. This category of non-Hodgkin's lymphomas exhibits considerable morphological, immunological, and clinical diversity and is generally considered to be a high-grade malignancy. In the present study, paraffin-embedded biopsy specimens of lymph nodes from 31 patients with PTCL who were treated with curative intent were evaluated by flow cytometry for DNA ploidy and proliferative activity (PA). DNA ploidy was not predictive of the clinical outcome. However, low PA, defined by less than or equal to 10% of cells in S + G2M phase of cell cycle, was associated with a favorable prognosis. Patients with tumors having low PA had a significantly higher complete remission rate (100%) as compared to those with high PA (55%; P less than 0.02), and the predicted actuarial 4-year survival of those with low PA was 85% versus only 50% for those with high PA (P less than 0.04). This is the first report of the effects of PA and DNA ploidy in patients with PTCL who were treated with curative intent. Additional studies of similar patients are needed to confirm these findings.