In order to assess the effect of body weight on cyclo-sporine disposition, 45 adult uremic candidates for renal transplantation underwent detailed nutritional assessment and pharmacokinetic analysis. There were 10 obese and 35 nonobese patients defined as actual body weight (ABW) >125 Per cent of ideal body weight (IBW), and arm fat area > 90th percentile. There was no significant difference in demographic variables such as age, sex, number of diabetics, IBW, serum lipids, or liver function tests between the 2 groups. Although there was a significant difference in ABW, pharmacokinetic analyses failed to demonstrate significant differences in bioavailability, elimination half-life, clearance, or apparent steady state volume of distribution when these calculations were normalized by IBW, body surface area, or as absolute values. Multiple stepwise linear regression failed to demonstrate a significant correlation between serum lipids or body size measurements and these parameters. When dosed according to ABW, obese recipients of renal allografts had a mean serum RIA trough level of 227 ng/ml as compared to 121 ng/ ml in nonobese recipients on day 7. Therefore in order to achieve comparable drug concentrations in the early transplant period, CsA should be given to obese patients based on their IBW.