The complications and causes of death of 105 patients with B cell chronic lymphocytic leukaemia followed for a median period of 5.5 years are described. Infection and secondary primary malignant tumours were the most common complications and also caused most deaths. S. pneumoniae, S. aureus, S. haemolyticus, E. coli and the zoster-varicella virus accounted for most infections and the lungs, skin and urinary tract were the sites affected. Even trivial infections were potentially serious. Haemolytic anaemia and vascular complications were also common. Older patients tended to have a shorter survival than the mean for the group whereas younger patients fared better. The mean survival was 6.2 years. Analysis confirmed the prognostic value of Rai staging. Advancing disease increased the liability to major infection. Light bone marrow infiltration five years post diagnosis indicated a good prognosis and preservation of normal immunoglobulin levels seemed beneficial. Immunoglobulin deficiency is the factor that correlates best with the frequency, severity and pattern of infection. Early stage disease provides a distinct benefit and there may be advantages in prompt diagnosis, regular assessment by immunoglobulin levels and bone marrow pattern and treatment on the first evidence of advancing disease. Fresh symptoms should be investigated in their own right because of the likelihood of second tumours.