The 14C-oxalate clearance was determined in 13 healthy subjects and 22 patients with various diseases and varying degrees of renal function impairment, including 5 patients with primary hyperoxaluria (PH). The clearances of oxalate (Cox) and creatinine (Ccr) 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 Cox is constant, irrespective of the underlying disease and the degree of renal failure. Plasma oxalate (Pox), as calculated from the urinary oxalate excretion (Uox) and Cox, was elevated in patients with severely impaired kidney function and those with PH. Plasma creatinine (Pcr) and Pox were correlated as well (r = 0.83). Pox 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 Uox and Ccr when a 14C-oxalate clearance test cannot be performed. The reasons for the constancy of the Cox/Ccr ratio are discussed, and it is suggested that the effective renal plasma flow (ERPF) is the regulating factor for the tubular secretion of oxalate.