The distributions of serum creatinine levels and their relationship with selected anthropometric and cardiovascular risk variables have been described in 3983 children and young adults, aged 5 to 26 years, obtained from a biracial population, Bogalusa, Louisiana. For both blacks and whites serum creatinine levels increased slowly with age, until 11 years of age when a steeper increase occurred to around 19 years of age. Before this age there was a significant age by sex interaction (p less than 0.0001) in the distributions of serum creatinine. After the age of 19 years mean serum creatinine levels were significantly higher in black men than in white men (1.16 mg/dl vs 1.09 mg/dl, p less than 0.0005) and in black women than in white women (0.87 mg/dl vs 0.84 mg/dl, p less than 0.08). Creatinine levels were also significantly higher in men than in women (1.11 mg/dl vs 0.85 mg/dl, p less than 0.0001). Creatinine clearance, estimated from an equation using serum creatine, age, and weight, showed a steady increase until 13 years of age when a maximum range of 120 to 140 ml/min was reached. By the age of 19 years the clearance had declined to a relatively constant range of 100 to 120 ml/min. Black men had the highest correlation of serum creatinine concentration with height, weight, and lean body mass after adjusting for age. White men had the highest correlation of serum creatinine concentration with uric acid. Diastolic blood pressure becomes an important determinant of creatinine levels by the age of 11 years. The race-sex differences in serum creatinine levels in children and young adults are likely related to body mass.