Cryoproteins are proteins precipitating at low temperature. Usually, the precipitate contains immunoglobulins (Igs), and are therefore called cryoglobulins. Very rarely, Igs do not precipitate, but, upon cooling, form a gel. Here, we report a case of cryogel observed in a patient presenting with Waldenström's disease. Using proteomic tools, a monoclonal IgM was identified as being the cause of the gel formation. Furthermore, addition of H(2)O before incubation at 4 degrees C demonstrated that the monoclonal IgM was precipitable as a type I cryoglobulin (hypocryoglobulin).