Lymphoproliferative disorders, especially chronic lymphocytic leukaemia (CLL), non-Hodgkin's lymphomas, Hodgkin's lymphoma and multiple myeloma are regarded as a hereditary entity with pleiotropic clustering in families, although the genuine alleles have not been found so far. The world-wide highest incidence of CLL and the existence of a systematic cancer registration since 1943 make Denmark a perfect place for epidemiological and genealogical investigations in the search for the genetics of the lymphoproliferative disorders. In Scandinavia, we see no signs of anticipation but marked linkage between parents and children, where the combination CLL in parent and CLL in child is more predominant than CLL in parent and a child with any other type of lymphoproliferative disorder. This same conservative pattern is also seen in parent-children transportation of non-Hodgkin's lymphomas and Hodgkin's lymphoma. That no certain linkage to other cancers can be significantly detected is discussed. A non-Mendelian mode of inheritance seems not unlikely in the familial clustering of the lymphoproliferative disorders.