The <sup>14</sup>C-oxalate clearance was determined in 13 healthy subjects and 22 patients with various diseases and varying degrees of renal functional impairment, including 5 patients with primary hyperoxaluria (PH). The clearances of oxalate (C<sub>ox</sub>) and creatinine (C<sub>cr</sub>) were correlated (r = 0.95). The regression line intersects the ordinate at the origin, while the regression coefficient is 2.0. This implies that the fractional C<sub>ox</sub> is constant, irrespective of the underlying disease and the degree of renal failure. Plasma oxalate (P<sub>ox</sub>), as calculated from the urinary oxalate excretion (U<sub>ox</sub>) and C<sub>ox</sub>, was elevated in patients with severely impaired kidney function and those with PH. Plasma creatinine (P<sub>cr</sub>) and P<sub>ox</sub> were correlated as well (r = 0.83). P<sub>ox</sub> values of patients with PH were above the 95% confidence limits of the regression line. It is of practical importance that Pox can be estimated from U<sub>ox</sub> and C<sub>cr</sub> when a <sup>14</sup>C-oxalate clearance test cannot be performed. The reasons for the constancy of the C<sub>ox</sub>/C<sub>cr</sub> ratio are discussed, and it is suggested that the effective renal plasma flow (ERPF) is the regulating factor for the tubular secretion of oxalate.