AbstractA patient with osteosclerotic myeloma and POEMS syndrome, unresponsive to pulse prednisone and melphalan therapy, was admitted to the hospital for a trial of plasma exchange therapy. The presentation included IgG lambda monoclonal gammopathy, peripheral neuropathy, hepatosplenomegaly, hyperpigmentation and thickening of the skin, edema, and tense ascites. Laboratory tests confirmed hypothyroidism, hypogonadism, and adrenal insufficiency. Six exchange procedures failed to affect the clinical course, and the patient died. Greater‐than‐one‐plasma‐volume exchanges (patient's measured plasma volume, 2,703 cc) were performed. When IgG and cholesterol removal were compared to the predicted removal, based on the volume of plasma removed, significantly less reduction in concentration than predicted was measured. IgG concentrations increased postapheresis and, at 2 weeks, three‐fourths of the removed IgG had reaccumulated. A reduced efficiency of removal of both IgG and cholesterol can be explained by postulating increased vascular permeability with free exchange of soluble substances from one compartment to another. If an abnormal product is produced by the disease and is responsible for the clinical syndrome, a more intensive schedule of plasma exchange therapy may be needed to achieve a sustained depletion of the responsible soluble substance. Alternatively, neither increased vascular permeability or the clinical manifestations are responsive to removal of a soluble substance or are caused by a soluble substance produced by the m