Black race and the absence of diabetes are associated with higher levels of serum creatinine in patients with end-stage renal disease. We examined whether these factors have a similar influence on creatinine excretion in men with chronic renal insufficiency. The hypotheses were tested in one sample (group A, n = 35) and the findings replicated in a second, independent sample (group B, n = 66). Creatinine excretion normalized to weight (UCr/kg) was compared by race and diabetic status. UCr/kg and creatinine clearance also were compared with the values predicted by the Cockcroft-Gault (CG) formula (based on the regression equation, UCr/kg = 28 - age/5). In each sample, mean UCr/kg was significantly higher in black patients than in nonblack patients (group A, P = 0.004; group B, P = 0.029), and UCr/kg and creatinine clearance were significantly underestimated by the CG predictions in black patients (group A, P < or = 0.004; group B, P < or = 0.019), but not in nonblack patients. Diabetes did not significantly influence UCr/kg. The analysis also was performed at two age levels (< 50 years or > or = 50 years) using groups A and B combined. For black patients younger than 50 (n = 10), observed UCr/kg (P = 0.059) and creatinine clearance (P = 0.025) exceeded the values predicted by the CG formula; the analysis of nonblack patients younger than 50 years was limited by sample size (n = 1). For patients aged 50 years and older (black, n = 32; nonblack, n = 58), mean UCr/kg was significantly higher in black patients (P = 0.034), and UCr/kg and creatinine clearance were significantly underestimated by the CG predictions in black patients (P < or = 0.002) but not in nonblack patients. In multiple regression analysis of all patients aged 50 years and older, UCr/kg was independently influenced by both race (P < 0.05) and age (P < 0.04) (overall model, multiple R = 0.31; P = 0.012). The prediction equation was UCr/kg (mg/kg) = 23.6 - age/8.3 + 1.9 x race (race = 0 if nonblack; race = 1 if black). We conclude that the creatinine excretion rate was strongly affected by race but not diabetes in men with chronic renal insufficiency. The CG formula significantly underestimated UCr/kg and therefore creatinine clearance in black patients. These findings may reflect differences between black and nonblack subjects in body composition, muscle metabolism, or diet, and the interaction of these factors with chronic renal insufficiency.