We studied 86 bone marrow biopsies (BMB) from 58 patients presenting with primary splenic marginal zone lymphoma (PSMZL). In 42 patients, a splenectomy was performed which enabled a histopathological diagnosis. In these patients, 44 biopsies were carried out before, and 25 after, splenectomy. In 16 recently observed patients, 17 BMB led to PSMZL diagnosis, and these patients were treated without splenectomy. Seven different patterns of infiltrates were recognized: intravascular, interstitial, nodular, massive, plasmacytic mimicking myeloma and transformation into large B-cell lymphoma (DLBCL). The association of an intravascular infiltrate and nodules with a germinal centre and/or a marginal zone favoured a diagnosis of MZL. Immunohistochemistry demonstrated the expression of B cell-associated antigens and, in 40% of the patients, a monotypic lymphoplasmacytic cell component. These patients often presented a serum M component and autoimmune disorders. In the past, such cases have been diagnosed as lymphoplasmacytic lymphoma. BM involvement was present in all patients. Successive biopsies showed progression and, after chemotherapy, a slight decrease in infiltrates. Transformation into DLBCL occurred in 11 of 34 patients. The patterns described are not specific for PSMZL and occur also in primary nodal MZL and, more rarely, in MALT-type lymphoma.