This study evaluates the correlation between estimated renal function parameters (ie, creatinine clearance by the Cockcroft-Gault and Lott-Hayton formulas, serum creatinine and blood urea nitrogen) and 24-hour creatinine clearance in 15 young and 15 elderly subjects. Correlation coefficients (r) for the elderly group comparing Cockcroft-Gault and Lott-Hayton against 24-hour creatinine clearance were 0.73 (P less than .005) and 0.60 (P less than .02) respectively, and r for the young subjects were 0.37 (P greater than .05) and 0.57 (P less than .05), respectively. In 13 elderly subjects with creatinine clearance of 60 mL/minute or less, four (31%) had blood urea nitrogen of 20 mg/dL or less and 13 (100%) had serum creatinine between 1.0-1.5 mg/dL. We conclude that the formulas of Cockcroft-Gault and Lott-Hayton are valid in ambulatory elderly patients who are functionally independent without severe underlying disease, have normal body weight and are not on medication affecting renal function. Furthermore, in this select elderly population, an apparently normal serum creatinine (1.0-1.5 mg/dL) and blood urea nitrogen (20 mg/dL or less) may frequently represent a 24-hour creatinine clearance of 60 mL/minute or less.