To confirm and expand previous observations about the association of monoclonal gammopathies with hemostatic defects, a prospective evaluation was made in 42 patients with lymphoplasmacytic disorders. The incidence of bleeding complications was low, despite the diversity of abnormal hemostatic tests observed in these patients. Patients with myeloma frequently had abnormal coagulation tests, including thrombin time (64%), fibrin degradation products (32%), platelet aggregation tests with different agonist (30% to 55%), and bleeding time (22%). The lack of platelet response to ristocetin and normal ristocetin cofactor activity in four patients with myeloma may suggest a Bernard-Soulier-type defect. Serum viscosity was negatively correlated with platelet aggregation with collagen, ristocetin, and adenosine diphosphate. In patients with immunoglobulin myeloma, there was a positive correlation between an increased viscosity and a prolonged thrombin time. This study demonstrates the wide variety of coagulation abnormalities in lymphoplasmacytic disorders, usually without significant clinical implications.