Serum immunoglobulin levels were periodically determined in 70 CLL patients and the changes were correlated with several clinical and laboratory parameters. It was found that the IgG and IgA levels decreased significantly as the disease progressed. A low IgG concentration was found at the time of diagnosis in 18.7% and after six years in about 50% of the patients. The IgM concentration, although initially low, increased during the follow-up in 43% of the patients and in four of them a monoclonal fraction appeared in the serum. The changes in the immunoglobulins did not correlate with age, sex or initial leukocyte count. Stage O patients as well as untreated patients also had a decrease in their immunoglobulin levels but advanced disease stage and especially continuous chemotherapy seemed to augment the drop in the immunoglobulin levels. Neither the initial immunoglobulin levels nor the subsequent changes, absolute or relative, had a significant prognostic value.