Division of Nephrology, S. Carlo Hospital, Milano, ItalyCryoglobulinaemia is a pathological condition in whichthe blood contains immunoglobulins that have theproperty of reversible precipitation in the cold.According to the classification based on the chemistryof the cryoglobulins involved there are three types ofcryoglobulinaemia. In type I the cryoprecipitatedimmunoglobulin, which can be found in patients withmultiple myeloma, Waldenstrom macroglobulinaemia,or idiopathic monoclonal gammopathy, is a singlemonoclonal immunoglobulin. Type II and III cryoglo-bulinaemias are both mixed types with at least twoimmunoglobulins; in both a polyclonal IgG is boundto another immunoglobulin, which is an antiglobulin,i.e. it acts as an anti-IgG rheumatoid factor. Theimportant difference between these two types of mixedcryoglobulinemias is that in type II the antiglobulincomponent, which is usually of the IgM class, ismonoclonal, while in type III it is polyclonal.A large percentage of the mixed cryoglobulinaemias,which constitute 60-75% of all types, are found inpatients with connective-tissue disease, infectious orlymphoproliferative disorders, hepatobiliary diseases,or immunologically mediated glomerular diseases, andcan therefore be considered 'secondary mixed cryoglob-ulinaemias'. However, in approximately 30% of allmixed cryoglobulinaemias, no underlying or associateddisease is found and the disease is referred to as'essential'.The clinical syndrome of essential mixed cryoglobuli-naemia (EMC) is characterized by purpura, weakness,arthralgia, and in some of the patients by glomerularlesions. Many subsequent reports have characterizedthis syndrome better. They have confirmed that thesyndrome can be associated with both type II andtype III cryoglobulins. In the rheumatological surveys,patients with type IN outnumbered those with type IIEMC. On the contrary, the surveys based on thedescription of the renal involvement documented alarge prevalence of type II EMC, the monoclonal IgMcomponent being nearly always an IgMk. While in thefew cases of type III EMC with renal involvement theglomerular lesions were variable and non-specific, intype II EMC, in which IgMk was the monoclonalcomponent, a specific well-characterized pattern ofimmune-complex-like glomerular disease has beendescribed in recent years, called 'cryoglobulinaemicglomerulonephritis' [1].It has always been suspected that in essential mixedcryoglobulinaemia the polyclonal IgG, to which poly-clonal IgM (type III EMC) or monoclonal IgM (type IIEMC) binds, is already bound to an antigen to formimmune complexes, so that IgM acts as an anti-immune complex antibody [1]. However, up to the endof the 80s, no single antigen that might have anaetiological role could be detected in the serum and/orcryoprecipitate of patients with this disease. Exposureto hepatitis B virus (HBV) had been documented bysome investigators many years ago [2], but this findinghas not been confirmed in large series in the same [3]as well as in other geographical areas [4], More recentlywe have suspected an ongoing Epstein-Barr virus(EBV) infection in patients with type II EMC becauseof the presence of IgM antibodies against the viralcapsid, and in some cases of the EBV genome in thepatients' lymphocytes [5].In the last 2 years many groups of investigatorsfrom Europe and USA [6-11], using an enzyme-linkedimmunoabsorbent assay and an even more sensitiverecombinant immunoblot assay (RIBA), have demon-strated a very high rate of positivity of anti-hepatitisC virus (HCV) antibodies in the serum of patients withall types of mixed cryoglobulinaemia (essential andsecondary), but not type I cryoglobulinaemia.(Table 1). In Type II EMC, three groups of investig-ators [9-11], using reverse transcription and DNAamplification, have documented an even greater preval-ence of HCV RNA, suggesting a consistent rate offalse-negative serological tests with the two assays usedto detect HCV antibodies, and demonstrating activeviral replication. Finally, a study of the cryoprecipitate[7,10,11] showed a lower prevalence of HCV antibodiesin cryoglobulins than in serum. However, quantitativeanalysis of these antibodies in serum and in dissociatedIgG isolated from the cryoprecipitate showed anincreased concentration in the cryoprecipitate [11].Elimination of IgM rheumatoid factor from the cryop-recipitate with dithiothreitol increased the prevalencerate of HCV antibodies from 41% to 94% in the studyof Misiani et al. [10], suggesting that HCV antibodies,albeit concentrated, remain hidden in the cryoprecipit-ate. Agnello et al. [11] demonstrated that more than99% of HCV RNA in serum from their patients withtype II EMC was cryoprecipitated, and that this cryop-