The significance of bone marrow involvement in patients with nodular lymphocyte predominant Hodgkin lymphoma is unknown. Of 275 patients diagnosed as lymphocyte predominant Hodgkin lymphoma at our institution (1983-2003), we identified 7 patients with purely nodular disease in the diagnostic lymph node biopsy specimen who also had bone marrow involvement. The latter was detected at the time of initial diagnosis in four patients, after one cycle of chemotherapy in one patient, and at relapse in two patients. There were six men and one woman with a median age of 37 years (range, 25-47 years). In all cases, the bone marrow was involved by large B cells, representing <10% of all cells, associated with a prominent T-cell and histiocytic background. All patients had laboratory, radiologic, and/or morphologic evidence of aggressive disease at the time of detection of bone marrow involvement. At last follow-up, four patients had died of their disease and three were alive following therapy. In conclusion, a small subset of patients in whom lymph node biopsy shows nodular lymphocyte predominant Hodgkin lymphoma with a purely nodular pattern also may have lymphoma in the bone marrow. Bone marrow involvement is associated with laboratory, radiologic, or morphologic evidence of aggressive disease and poor prognosis. Although the best terminology for these bone marrow lymphomas is uncertain, the aggressive clinical behavior of these neoplasms supports the need for intensive therapy.