Chronic renal failure in man is associated with hyperglucagonemia that is not corrected by hemodialysis. Plasma glucagon concentrations were measured in nine patients before and after renal transplantation. Mean plasma glucagon concentration in eight patients with chronic renal failure before transplantation was 295 plus or minus 171 pg/ml (plus or minus SD). After successful transplantation, mean plasma glucagon concentration fell to 134 plus or minus 81 pg/ml (plus or minus SD) (P less than 0.001). Plasma glucagon concentration remained elevated in an additional patient who received a cadaveric graft that never functioned. Immunologic rejection of transplanted kidneys was associated with a dramatic increase of plasma glucagon concentration.