According to some findings [Ross et al. 1987], the plasma concentration of creatinine (Pcr) is an inaccurate reflection of the glomerular filtration rate (GFR) in renal graft recipients with stabilized renal function, treated with cyclosporine A (CyA). In this study, we sought to determine whether the inaccuracy of GFR assessment on the basis of Pcr or creatinine clearance (Ccr) in these individuals is greater than in patients suffering from various chronic renal diseases untreated by any drugs during the examination. The study was performed in 30 renal graft recipients, treated with CyA in combination with azathioprine and prednisone. Further, 51 patients suffering from a chronic renal disease, mostly chronic glomerulonephritis or tubulointerstitial nephritis, were investigated. GFR was evaluated on the basis of polyfructosan clearance (CPF). A significant linear relation between Ccr and CPF (r = 0.829, p less than 0.001) was demonstrated in individuals with a transplanted kidney graft treated with CyA. A relationship of the same character was observed in the group of patients suffering from chronic renal diseases (r = 0.935, p less than 0.001). There is no statistically significant difference between the regression lines characterizing these relationships in both groups. A significant correlation of hyperbolic character between Pcr and CPF was found in both groups investigated (r = 0.693, p less than 0.001, and r = 0.741, p less than 0.001, respectively). The hyperbolic relationship noted in the studied groups did not differ significantly. These findings confirm a normal, or a mildly raised Pcr can be associated with a marked decrease in GFR, in some cases to a value as low as a third of the normal one.(ABSTRACT TRUNCATED AT 250 WORDS)