The objective of the current study was to determine the best index of residual glomerular filtration rate (GFR) by comparing simultaneously measured clearances of inulin, 125I-iothalamate, endogenous creatinine, urea and 169ytterbium diethylenetriaminepentaacetic acid (169YB-DTPA) in patients receiving repetitive hemodialysis. In patients with GFR less than 5 ml/min but greater than 1 ml/min, 125I-iothalamate clearance showed the best correlation with inulin clearance. However, creatinine clearance correlated better with inulin clearance than urea clearance and as well as urea + creatinine/2. In the patients with measured GFR less than 1 ml/min, the correlation of 125I-iothalamate, creatinine, urea and urea + creatinine/two clearances with inulin clearance was satisfactory. Similarly, satisfactory correlations were obtained when the relationships were examined across the entire range of measured clearances less than 5 ml/min. A simple, practical method is descriged for the accurate serial measurement of residual GFR in patients receiving repetitive dialysis.