The objective of this study was to define the normal urinary creatinine clearance and its determinants. The creatinine clearance was measured in both sexes in a large random Belgian population sample (BIRNH: Belgian interuniversity Research on Nutrition and Health) by measuring serum creatinine and the 24 h urinary creatinine excretion. At a mean age of 50.8 years in 2,075 men and 50.1 years in 1,933 women the mean creatinine clearance was 101 +/- 30.5 and 86.9 +/- 25.8 ml/min respectively. In multiple regression analysis the creatinine clearance in both sexes correlated negatively with age and positively with weight and 24 h urinary sodium, potassium, calcium and magnesium (all P < 0.0001). Total R2 was 0.40 in men and 0.35 in women. After adjustment for age, height and weight no significant relationship exists between the 24 h urinary creatinine clearance and either systolic or diastolic blood pressure. Serum creatinine increased with age, more so in women, and was at all ages higher in men compared to women. The creatinine clearance decreased with age in both sexes. Our study demonstrates the existence of highly significant associations between urinary cations and the creatinine clearance, independent of total caloric intake.