The plasma of a patient with myasthenia gravis had strong lupus anticoagulant activity and his IgM paraprotein displayed non-specific inhibition to coagulation factors IX, XI, XII, prekallikrein, and high molecular weight kininogen. He was placed on prednisolone, which resulted in improvement in his myasthenic symptoms, but the prolongation of APTT and macroglobulinemia remained. Double filtration plasmapheresis successfully decreased the serum IgM level from 1,190 mg/dl to 375 mg/dl and APTT improved from 58 s to 38 s. Myasthenia gravis is frequently associated with other autoimmune diseases, but the association with lupus anticoagulant and IgM gammopathy is rare.