ABSTRACT. End-stage renal disease is an important and costly health problem. Strategies for its prevention are urgently needed. Knowledge of the population-based prevalence of renal insufficiency in nondiabetic adults would inform such strategies. Black and white nondiabetic adult participants in the Third National Health and Nutrition Examination Survey were analyzed. The analysis was stratified by age, gender, and race, and four clinically applicable methods were used to assess renal function. There were 13,251 complete records for analysis. By the Modification of Diet in Renal Diseases (MDRD) GFR (GFR) prediction Equation 7, 58% (95% confidence interval [CI], 56 to 60%) of the total adult nondiabetic black and white US population had MDRD GFR below 80 ml/min per 1.73m2, 13% (95% CI, 11 to 14%) below 60 ml/min per 1.73m2, and 0.26% (95% CI, 0.19 to 0.33%) below 30 ml/min per 1.73m2. By the Cockcroft-Gault formula, the equivalent figures were 39% (95% CI, 37 to 41%), 14% (95% CI, 12% - 16%), and 0.81% (95% CI, 0.46 to 1.2%), respectively. The findings of an unexpectedly high prevalence of low clearance and the increased prevalence of low clearance with age were consistent across the four clearance estimation methods used and for each race-sex stratum. The absolute magnitude of the prevalence of low clearance was, however, dependent on the clearance method used. Assessed by estimation from serum creatinine, low clearance may be very common, particularly with advancing age. The prognosis (in terms of risk for progression and end-stage renal disease) of low clearance in unreferred populations may differ from that in referred populations and requires further study.