Significantly higher levels of immunoconglutinin were found in the sera of 40 Cameroonian patients aged between 15 and 80 years with congestive cardiomyopathy than in a matched group of normal Cameroonian controls. C3 values were also abnormal in the group with congestive cardiomyopathy, and a negative correlation was found between the C3 and immunoconglutinin values, indicating complement incorporation into immune complexes. Further, the levels of immunoglobulins G, M, and A were all raised, the most striking increase being of IgM, which was greater than in the normal controls and in a group with 'other cardiac disease' studied for comparison. Complement-fixing autoantibodies to cardiac muscle of the class IgG and IgM were found by immunofluorescence techniques in a significant proportion of the patients with congestive cardiomyopathy. Finally, evidence of trypanosomiasis was found by immunofluorescence tests in 27-5 per cent of patients with congestive cardiomyopathy, compared with 8-5 per cent of patients with other cardiac disease, and 1-9 per cent of normal Cameroonian controls selected randomly. We think that some cases of congestive cardiomyopathy in Cameroon may be caused by untreated subclinical attacks of African trypanosomiasis which produce immune complexes that damage the cardiac tissue where the parasite is lodged. Secondary autoimmune carditis modifies the course of the disease, with the resulting end-stage picture.