Renal function studies 12 to 41 months after transplantation have been performed on seven cadaver renal allograft recipients who demonstrated immediate primary function after transplant (group A) and seven similar recipients who had delayed primary function (group B). The groups were matched as closely as possible for major physical characteristics and their postoperative management; in particular, only one patient had a post-transplant renal biopsy. Glomerular filtration rate was determined by 24-hr creatinine clearance, endogenous creatinine clearance, and inulin clearance was usually lower in those patients having delayed primary function and they excreted more glucose per 24 hr and reabsorbed a smaller proportion of the filtered glucose load under infusion conditions. These results are discussed in relation to the effect of immediate and delayed primary function on the long-term prognosis of such patients.