Study Objective. To evaluate the bias and precision of three methods of measuring glomerular filtration rate (GFR) relative to a standard method. Design. Prospective, outpatient study. Setting. University-affiliated general clinical research center. Patients. Twenty-six patients with various degrees of renal function (GFR range 25–151 ml/min/1.73 m2). Interventions. Each patient received iothalamate twice during the study visit, first as a bolus injection and then as a priming dose followed by a constantrate infusion for 2.5 hours. Measurements and Main Results. Plasma (ClpIVB) and renal clearances (ClrIVB) after bolus injection and plasma clearance during constant-rate infusion (ClpINF) were compared with standard renal clearance during constant-rate infusion (ClrINF). All three measures were highly correlated with ClrINF (r>0.90, p<0.001). The mean ClrIVB was not significantly different from ClrINF (106.3 ± 30.4 vs 104.2 ± 28.5 ml/min/1.73 m2) and provided a precise (8.8%, 95% CI 6.5–11.1%) and unbiased measure of GFR. Both ClpIVB and ClpINF were positively biased; values exceeded ClrINF by 11.8 ± 11.1 (p=0.0001) and 10.5 ± 12.5 ml/min/1.73 m2 (p=0.0003), respectively. Use of a nonrenal correction factor of 9.8 and 10.5 ml/min/1.73 m2 for infusion and bolus plasma clearance values, respectively, eliminated bias and improved the precision of these methods. Conclusions. Iothalamate renal clearance after bolus injection is a simple, accurate, and precise measurement of GFR and may be a useful alternative to the standard infusion method in clinical investigations. The corrected plasma clearance provides a simple index of GFR for clinical practice.