Several patients with Paget's disease have been reported to have monoclonal IgM proteins in their serum. We have systematically studied 26 patients with Paget's disease severe enough to require diphosphonate therapy. Five of these patients were found to have isolated elevations of serum IgM which were well outside the normal range for their age, sex and race. An additional patient had elevations of both IgG and IgM. These patients seem to represent a subset of Pagetics. They do not differ from the remainder of the group with respect to severity or extent of disease, response to therapy, presence of intercurrent disease or age. They do have a different male to female ratio. These observations suggest that some patients with Paget's disease may have disordered immune regulation or that they may be undergoing a primary response to an agent involved in the aetiology or pathogenesis of the disease.