We report the clinicopathologic findings of 41 patients with Ki-1 (CD30)-positive large cell lymphoma. The median age was 50 years; 13 patients were under 40 years of age. Ten patients presented with extranodal disease. Fifty-five percent of the patients presented with stage I or II disease, and bone marrow involvement was histologically documented in 30% and occurred exclusively in patients over 40 years of age. Two cytomorphologically distinct groups of Ki-1--positive large cell lymphomas could be separated. Group A lymphomas consisted of pleomorphic large cells, sometimes with wreathlike and embryo-like nuclei, whereas group B lymphomas displayed a rather monomorphic appearance. Clinically the two groups of lymphomas differed with respect to stage of disease, frequency of bone marrow involvement, and median survival. On paraffin sections, the Ki-1--related antibody Ber-H2 provided excellent staining results in all cases. Immunologic phenotyping disclosed a T cell type in the majority of cases, revealed marked loss of differentiation antigens, and frequent expression of HLA-DR and IL-2 receptor. The overall median survival was 13 months. Age below 40 years, limited stage of disease (I and II), and, although not statistically significant, lymphoma morphology were associated with longer survival. We conclude, that Ki-1--positive large cell lymphomas represent a morphologically and immunologically heterogeneous category of hematolymphoid neoplasms derived from dedifferentiated and activated lymphoid cells with marked age-dependent prognosis.