A study of 260 patients with non-Hodgkin's lymphoma (NHL) who underwent bilateral bone marrow biopsy at initial diagnosis showed marrow disease in 99 (38%) cases. The highest incidence of disease (83%) was seen in small lymphocytic lymphoma (SLL) and the lowest (19%) in diffuse large cell lymphoma (DLCL). Among cases with positive marrows, disease was bilateral in all 15 cases of SLL but in only 10 of 20 (50%) of the DLCL cases. In 30 of 99 (30%) positive marrows disease was unilateral. Follicular lymphomas were strongly associated with a paratrabecular pattern, with 40 of 45 positive cases showing this. Discordant histology was seen in six of 20 positive cases of DLCL and two of 37 positive cases of follicular small cleaved cell lymphomas (FSCCL). A bone marrow aspirate was positive in only 56 of the 99 (57%) cases. Peripheral blood disease was present in 15% of the bone marrow positive cases and in 6% of the cases overall. The incidence of marrow disease varies with the histological subtype of lymphoma. The paratrabecular pattern is associated with follicular lymphoma, and bilateral biopsy specimens increase the positivity rate in most subtypes of NHL.