The authors have studied proliferation (growth fractions) in non-Hodgkin's lymphomas (NHL) using an immunohistochemical double staining technique with the monoclonal antibody (MoAb), Ki-67, in order to clarify the relationships between histologic type, proliferation and prognosis. The percentages of Ki-67 positive (Ki-67+) cells in B cell lymphomas were higher for diffuse lymphomas than for follicular lymphomas and increased in order from small to large cell types. In addition, the percentage of Ki-67+ cells in B cell lymphomas inversely correlated with survival in months (n = 33 r = -0.54 p < 0.01). These results indicate that the percentage of Ki-67+ cells in B cell lymphomas correlates with histologic type and prognosis. Although the prognosis of T cell lymphomas is considered worse than that of B cell lymphomas, the percentage of Ki-67+ cells was lower, in general, in T cell lymphomas than in B cell lymphomas. These data indicate that proliferation in T cell lymphomas does not correlate with survival.