We evaluated the incidence of bone marrow involvement in 121 patients with non-Hodgkin's lymphoma who were seen prior to the institution of definitive therapy. Involvement of the marrow was found to be dependent on both histologic type and the extent of extramedullary disease. It was observed most frequently in patients with poorly differentiated lymphocytic lymphoma (60%), but was not observed in 36 patients of all histologic types whose clinical evaluation and/or laparotomy revealed disease in stage I or II. Among 56 patients with poorly differentiated lymphocytic lymphoma in stage III or IV exclusive of marrow involvement, disease was observed in the marrow in 40 patients, or 71%. These observations, together with the results of previously reported therapeutic trials, suggest that staging laparotomy and total nodal radiotherapy alone may be of limited value in patients with poorly differentiated lymphocytic lymphoma when clinical evidence of stage III or IV disease is present.