The purpose of this study was to examine the variabilityof 24-hour urinary and serum creatinine levels andcreatinine clearance in normal subjects and to developnomograms for assessing the adequacy of 24-hour urinecollections. The data were from a longitudinal researchprogram examining biochemical, hormonal, and hemodynamicparameters in normal subjects. Bloods and 24-hoururine specimens were collected at yearly intervals from144 people over 9 years, and from an additional 110 over4 years. The subjects were originally distributed equallyby sex, race (black, white), blood pressure (three groupswithin the normal range), and age (three groups). Men had 33% higher urine creatinines per weight thanfemales (P < 0.001). Because they only had 8% highercreatinine clearance per weight they also exhibited 21%higher serum creatinine. Blacks had 5% higher urinecreatinine per weight than whites, perhaps reflectinggreater muscle mass, but their serum creatinines werenot different from those of whites, reflecting a 5% highercreatinine clearance by weight than whites (P < 0.01).Interestingly, older black men (age >60 years) had 12%lower urine creatinine/weight than younger black men (P < 0.001). They also had 13% lower creatinine clearanceby weight, resulting in no net difference in serumcreatinine. The intraindividual variability in urine creatinineexcretion averaged 15% and did not differ between blacksand whites and men and women. The within individualvariability in serum creatinine and creatinine clearanceaveraged 14 and 20%, respectively. Nomograms of therelationship of the 24-hour urinary creatinine to weightwere developed for men and women, blacks and whites.They should be useful in future studies for assessing thecompleteness of 24-hour urine collections. Altogether the results show that blacks produce morecreatinine than whites, but they compensate for that withincreased creatinine clearance. Men produce morecreatinine than women but only have a modestly highercreatinine clearance, which results in a higher serumcreatinine. The higher creatinine clearance of blacks maybe the result of a difference in renal function betweenblacks and whites. Am J Hypertens 1988; 1:124-131