Pretreatment lymph nodes, bone marrow, and blood were examined in 176 cases of non-Hodgkin's lymphoma. By the criteria of the Lukes and Collins functional--morphological classification, 158 (90%) were B-cell lymphomas and 17 (10%) were T-cell lymphomas. Bone marrow involvement was present in 53% of cases: 51% of B-cell types and 65% of T-cell types. Marrow involvement was most frequent in small lymphocyte (B) (89%), convoluted lymphocyte (60%), and small cleaved follicular center cell (FCC) lymphomas (55%). The pattern of bone marrow involvement was most frequently focal paratrabecular in B-cell lymphomas and diffuse in T-cell lymphomas. Blood involvement was present in 50% of cases with bone marrow lymphoma and generally reflected extensive bone marrow disease. There was a higher incidence of both bone marrow and blood involvement in pediatric patients than in adults.