In this report, we analyze the clinical, morphologic, and immunologic findings of 19 patients with Ki-1+ large cell lymphoma, a recently described malignant lymphoma which is usually of T-cell phenotype. Most patients in this selected series of 14 children and adolescents and five adults presented with peripheral lymphadenopathy or skin lesions. No patient had bone marrow involvement. Distinctive morphologic features of Ki-1+ large cell lymphoma are tumor cell pleomorphism, sinus infiltration, fibrosis, partial lymph node involvement, sparing of B-cell regions, and a prominent plasma cell infiltrate. Seventy-two percent of cases were of T-cell phenotype; the remaining cases expressed neither B- nor T-cell-specific markers. Virtually all cases were positive for Ia (HLA-DR), Tac (interleukin-2 receptor), and T9 (transferrin receptor), indicating that this lymphoma is frequently a tumor of activated T-cells. We conclude that a diagnosis of Ki-1+ large cell lymphoma should be considered in any pleomorphic tumor with the features described in this report.