Cryoglobulinaemias contain seric immunoglobulins which precipitate at low temperatures in some tissues. Based on immunohistochemical analysis of their components (monoclonal or polyclonal), 3 types of cryoglobulinaemias have been identified, and a classification has been proposed. The presence of a paraprotein within the cryoglobulinaemia is due to the proliferation of B cells. Type I is mainly associated with a lymphoproliferative syndrome (myeloma and others...) whereas type II (mixed cryoglobulinaemia) and type III are the result of viral and bacterial infectious diseases, and of autoimmune diseases. Over the last few years, infection by hepatitis C has been found in more than 90% of types II and III cryoglobulinaemias. Cryoprecipitates are pathogenic for kidneys, skin and nervous tissues. In some organs, they are responsible for specific histological lesions, such as necrotizing angeitis with vascular microthrombosis. In kidneys, they occur as endocapillary proliferative glomerulonephrites, accompanied by precipitation of immunoglobulins in the form of intracapillary thrombi. Precipitated immunoglobulins can be identified on frozen specimens. Ultrastructural studies confirm the fibrillar aspect which is characteristic of the cryoprecipitate.