The work was designed to assess the suitability of both measured endogenous creatinine clearance (CCR) and predicted creatinine clearance (P-CCR) to evaluate GFR in chronic renal disease (CRD) by utilizing the renal clearance of inulin (CIN) as gold standard. A total of 124 subjects were studied (62 healthy, 62 with CRF). CCR significantly overestimated GFR in healthy subjects as well as in CRF, whereas P-CCR was identical to GFR. The CCR/CIN ratio which calculates the fractional creatinine clearance and provides a rough estimation of the contribution of creatinine secretion in explaining the differences between CCR and GFR was increased in CRD and especially in CRD of glomerular origin. The ration P-CCR/CIN was significantly lower than CCR/CIN in healthy subjects and in patients with CRD of glomerular origin. The data are against the use of CCR in assessing GFR in healthy subjects and in patients with CRD.