The clinical course of 40 patients with significant quantities of mixed cryoglobulins, but without lymphoproliferative, collagen-vascular or chronic infectious diseases, is presented. These cases comprise 51.3 percent of all mixed and 31.7 percent of all types of cryoglobulins evaluated by us over the period 1960--1978. A characteristic clinical syndrome, consisting of recurrent palpable purpura (100 percent), polyarthralgias (72.5 percent) and renal disease (55 percent), was seen. Biopsy specimens of skin lesions showed cutaneous vasculitis, and half had immune reactants in vessel walls. Seventy percent of patients had evidence of hepatic dysfunction, often subclinical, and more than 60 percent of those tested had serologic evidence of prior infection with hepatitis B virus. Hepatic lesions ranged from minimal triaditis to chronic active hepatitis and/or cirrhosis. All 22 patients in whom clinical renal disease developed had significant proteinuria; 63.6 percent had diastolic hypertension, 77.3 percent edema, 45.5 percent renal failure and 22.7 percent were nephrotic. Glomerular disease associated with deposition of immunoglobulin G, immunoglobulin M and complement, often with coexistent renal arteritis, was confirmed pathologically in 15 cases. All cryoglobulins had rheumatoid factor activity and consisted of IgM and polyclonal IgG; five also contained IgA. Thirteen had a monoclonal IgM kappa component. Serum protein electrophoresis was unremarkable or showed diffuse hyperglobulinemia. Striking depression of early complement components was noted but did not correlate well with the cryoprotein concentration, renal involvement or clinical course. Follow-up for periods up to 21 years from onset of symptoms revealed that renal involvement has a deleterious effect on prognosis. Postmorten examinations of nine patients demonstrated widespread vasculitis in addition to renal involvement. Preterminal infection was found in eight.