In order to assess the impact of demographic factors on serum levels of cyclosporine (CsA) estimated by radioimmunoassay (RIA) in renal allograft recipients, 493 pharmacokinetic studies were performed in 212 patients. Neither the presence of diabetes mellitus nor the CsA dosing frequency affected the measured pharmacokinetic parameters. Age over 45 years led to slower CsA clearance with resultant increase in maximum serum concentration (Cmax) per administered milligram, and increased volume of distribution. Female patients showed more rapid drug clearance, but greater volume of distribution. Concomitant hepatic impairment reduced drug clearance, increasing the area under the curve (AUC) per administered milligram of drug, and the Cmax. Patients treated with a rapid steroid taper showed a shorter half-life and lower Cmax than those receiving a slow steroid taper. Nephrotoxicity was associated with increased AUC per administered mg, while patients with acute tubular necrosis requiring dialysis showed poorer drug absorption, lower Cmax, and longer time to peak. The only effect of cimetidine administration was a slightly shortened time to peak. Serial analyses posttransplant in 17 patients suggested a tendency toward improved drug absorption with no effect on other parameters. These studies demonstrating the significant impact of demographic factors thus afford a basis on which to predict the trend of anticipated CsA levels as measured by RIA in renal allograft recipients.