The frequency, pattern, extent and clinical features of bone marrow involvement in 94 cases of non-Hodgkin's lymphoma, classified according to the International Working Formulation, were reviewed. In 30% of cases there was bone marrow involvement by lymphoma at the time of diagnosis. Marrow involvement was most frequently found in the follicular small cleaved cell (57%), diffuse mixed small and large cell (56%), and lymphoblastic (56%) lymphomas. A paratrabecular pattern of marrow involvement tended to occur in low or intermediate grade lymphomas. High grade lymphomas tended to show an interstitial or diffuse pattern of marrow involvement. The results of marrow aspiration and biopsy were of equal value and complementary to each other. In 43% of cases with marrow lymphoma there was peripheral blood involvement at the time of diagnosis. Blood involvement was most frequently found in the lymphoblastic lymphoma (70%). In general, the degree of blood involvement was related to the extent of marrow involvement. Patients with marrow involvement often presented with "B" symptoms and hepatomegaly. Abnormalities in at least one of the blood counts were quite common (93%). A normal blood count made marrow involvement unlikely but not impossible. Other common laboratory findings included hyperimmunoglobulinemia (47%) and elevated serum levels of lactate dehydrogenase (56%). Patients with marrow involvement showed a significantly higher frequency of hepatomegaly and abnormal blood counts, as compared to those with negative marrows (p less than 0.05 and less than 0.001, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)