Renal transplants do not always function immediately and this non-function may be related to poor long-term graft survival. The causes of this primary non-function (PNF) are poorly understood. The incidence of PNF in 106 consecutive renal transplants was found to be 57 per cent. Using stepwise logistic regression analysis of 20 prospectively collected variables for each transplant two have been found to be positively associated with PNF of the transplant: (1) the use of cyclosporin as an immunosuppressant and (2) a second warm ischaemia (anastomosis) time of more than 30 min. It is proposed that the use of cyclosporin be restricted in the early post-transplant period until initial function of the transplant is established. The surgical procedure of kidney transplantation should be planned to minimize the anastomosis time prior to recirculation of the graft.