Following acceptance to a renal dialysis therapy program, 13 patients underwent serial examination of the serum complement system for periods of 6 to 12 months. Serum levels of C3 and C4 were examined at monthly intervals before the first dialysis of each month. The patients suffered from various primary renal diseases, including glomerulonephritis, obstructive uropathy with chronic pyelonephritis, nephrosclerosis and chronic nephropathy of undetermined nature. Levels of serum C3 were depressed at some time during the study period in all patients. Serum C4 levels were normal. The depression of serum C3 levels was significant and prolonged in nine patients. There seemed to be no relationship between the nature of the basic disease and the depression of serum C3 levels. Caution should be exercised in attributing depressed C3 levels to a particular type of underlying renal disease in patients on renal dialysis.