Morphologic and clinical features of 30 patients with peripheral T-cell lymphoma (PTCL) were studied with particular attention to bone marrow and blood manifestations. Twenty-four (80%) patients had marrow involvement with lymphoma in trephine biopsies at initial diagnosis; two other patients subsequently developed marrow involvement. The bone marrow lesions were diffuse in 58% of the cases and focal, nonparatrabecular in 42%. A morphologic spectrum of lymphoma cells was seen with cases classified into small cell, mixed cell, and large cell/immunoblastic lymphoma. The bone marrow lesions were characterized by a heteromorphous population of lymphocytes, prominent vascularity with endothelial cell proliferation, reticulin fibrosis, and a polycellular infiltrate composed of plasma cells, eosinophils, and histiocytes. The histopathologic features in bone marrow biopsies were not pathognomonic for PTCL; the differential diagnosis may include non-Hodgkin's lymphomas of B-cell type, polymorphous reactive lymphoid lesions, including those from patients with acquired immune deficiency syndrome (AIDS), angioimmunoblastic lymphadenopathy, Hodgkin's disease, and systemic mastocytosis. The patients ranged in age from 13 to 81 years (median, 61 years) and generally presented with constitutional symptoms, lymphadenopathy, and hepatosplenomegaly. Abnormalities in one or more hematologic parameters were common and, in general, related to the degree of bone marrow involvement. Hypocalcemia was found in 40% of the patients studied and hypercalcemia in 4%. The median survival for PTCL patients was 11 months. Patients with small cell lymphoma, large cell/immunoblastic lymphoma, and marked eosinophilia had the shortest median survivals.