The clinical course was reviewed of 102 renal allograft recipients between December 1967 and December 1973. Only 4 of 21 patients (19 per cent) who had 2 or more episodes of rejection during the first 2 months had a functioning graft at the end of 1 year, compared to 24 of 30 patients (83 per cent) who had no rejection episodes. A similar trend was seen 2 to 6 months after transplantation. During the first 2 years vigorous immunosuppressive therapy for rejection in the first few months resulted in 12 deaths (44 per cent) of 27 patients. Subsequent to this immunosuppressive therapy was modified and grafts were removed if there was not a prompt recovery of function after treatment, which resulted in a significant decrease in mortality rate to 16 per cent. There also was improvement in the over-all survival of patients with functioning grafts from 37 to 56 per cent. Serious complications and mortality could be related to high dosage of steroids and severe leukopenia. A white blood count of less than 1,000 mm.3 on 3 successive days was associated with a mortality rate of 52 per cent, compared to 15 per cent in those without leukopenia. Serious consideration should be given to early graft removal in patients who have 2 or more episodes of rejection in the first few months after transplantation, particularly when there is not a prompt improvement in renal function after immunosuppressive therapy. High doses of steroids (greater than 1 mg. per kg. for more than 26 days during the first 60 days) should be avoided to decrease morbidity and mortality rates from serious infections. The results of histocompatibility (HL-A) matching in 72 donor-recipient pairs indicated an improved graft survival when there was a match of 2 or more antigens, which is supported by the results recently reported by the Transplant Registry. The results of mixed lymphocyte reactions in 20 live donor-recipient pairs showed a marked improvement in graft survival when there was less than 20 per cent stimulation and it appeared that this reaction was of more important prognostic significance than the results of histocompatibility (HL-A) matching in these patients.