Primary lymphoma of the spleen is characterized by predominant splenomegaly. Lymphoplasmacytic malignant lymphoma of the spleen, of low malignancy in the Kiel classification, low and intermediate grade in the National Cancer Institute Working Formulation (NCIWF), is rare. It is often associated with a monoclonal immunoglobulin M (IgM). Four patients presenting with primary splenic lymphoma of plasmacytic type associated with a high level of monoclonal IgM and a lupus anticoagulant (LA) are described. This association has not previously been reported. In contrast with the usual heterogeneity of LA, this LA is relatively homogeneous with an important prolongation of the prothrombin time (greater than 18 sec for a control of 12), more prolonged partial thromboplastin time (PTT) of the mixture patient + control plasma than PTT of the patient plasma. Despite the important coagulation abnormalities, none of these four patients has presented any hemorrhagic or thrombotic complications, even during major surgery such as splenectomy. The lupus-like anticoagulant effect ran parallel with the monoclonal IgM. Survival, after splenectomy and chemotherapy, appears to be favourable: three patients are alive with survivals of greater than or equal to 7 years. The follow-up is as yet too short for the last patient.