Chronic cold agglutinin disease is an uncommon form of haemolytic anaemia. Traditionally the "idiopatic" form and the "secondary" form (seen in some cases of malignant lymphoma) have been regarded as two clearly different entities. We describe three patients with chronic cold agglutinin disease in a population of 100,000. They had been previously diagnosed as having idiopatic autoimmune haemolytic anaemia, but further investigations revealed lymphoproliferative bone marrow disease in all three cases. We discuss some pathogenetic features of cold agglutinin disease on the basis of our observations and the available literature. In conclusion, this disease represents a spectrum of clonal lymphoproliferative disorders rather than separate primary and secondary forms. The therapeutic principles are discussed. Corticosteroids should usually not be used to treat this disease.