Carcinoembryonic antigen (CEA) was estimated in plasma from 70 patients with a renal transplant, 105 patients with glomerulonephritis who had received immunosuppressive therapy, and 124 healthy controls. There were raised levels in 30% of those with a renal transplant, 10% of those with glomerulonephritis and 2% of controls, and levels were higher in current smokers. CEA levels did not correlate with pre-transplant dialysis time nor with serum creatinine levels, but tended to fall with increasing time after transplantation, especially in non-smokers. CEA levels did not correlate with prednisolone dosage nor with number of rejection episodes, after allowing for time after transplantation and smoking habit. Nine of 70 patients with a renal transplant and three of 105 with glomerulonephritis had cancer, of skin in seven, cervix uteri in four, and colon in one. CEA was raised in all four transplant recipients with a visceral cancer (cervix three and colon one), but in none of the five with cutaneous cancer. Raised CEA levels occurring late after a renal allograft should prompt a careful search for visceral cancer.