We studied clinical and immunological characteristics of 15 patients with chronic cold agglutinin disease (CAD). Mean age at disease debut was 68 years for female and 67 years for male patients. The patients had no signs of other autoimmune diseases. All patients had V(H)4-34 encoded IgM kappa cold agglutinins (CA) in high titre. In five patients IgM increased significantly with advancing disease. Seven patients had reduced concentrations of lymphocytes, largely of CD4 and CD8 T cells. Percentages of NK cells (CD56) and B cells (CD19) were increased in seven and three patients, respectively. In six out of nine patients a clonal expansion of kappa positive B cells was found. Serum C3 was decreased in nine patients and C4 was decreased in 11 patients, six of whom had reduced CH50. Such data indicate that patients with CAD experience a continuous low-grade complement consumption. Five patients had experienced increased haemolysis during infections. After addition of active complement to patient sera in vitro, six sera showed increased haemolytic activity. Our results indicate that some patients with CAD have a relative deficit of complement in their serum and that an increase of complement production occurs during an acute phase reaction which enhances haemolysis. Our data also indicate that both CA titre and thermal amplitude are important characteristics when predicting complement activation and clinical course in CAD.