We investigated the clinical, pathological, and immunological features of "idiopathic" cold agglutinin disease (CAD) in a population-based study. Fourteen patients were studied, giving a prevalence of about 14 per million with a mean age of 75 years. Haemolysis was present in all cases, but only eight patients had clinical symptoms of peripheral haemagglutination. Serum electrophoresis, immunofixation, morphological bone marrow evaluation, and flow cytometric immunophenotyping were used to detect any monoclonal lymphoproliferative disorder. Flow cytometry seemed to be a sensitive way to demonstrate a clonal B-cell proliferation. Some evidence of clonality was found in 13 patients, and a clonal lymphoproliferative disease was documented by flow cytometry or biopsy in 10 out of 11 patients. We conclude that CAD is a symptom-producing monoclonal lymphoproliferative disorder in nearly all patients.