Although albumin excretion rate is commonly estimated by using albumin/creatinine ratio (A/C), gender and race differences in creatinine excretion may bias this estimate. The authors optimize the use of an untimed (spot) urine specimen among 3,371 Blacks and Whites aged 28-40 years in the Coronary Artery Risk Development in Young Adults Study in 1995-1996. Using three 24-hour collections during the year 5 examination, they determined k = 0.68 x 0.88 in Black men, 0.88 in Black women, 0.68 in White men, and 1.0 in White women to reflect gender and race differences in creatinine excretion. The authors then computed A/C adjusted for race and sex differences in creatinine excretion (A/kC) by using an untimed urine sample in the year 10 examination. A/kC >or= 25 mg/g (194 cases of microalbuminuria and 26 cases of clinical grade albuminuria) was more common among Blacks (9.1%) than among Whites (4.2%) and among men (8.2%) than among women (5.0%). Use of the unadjusted A/C underestimated the prevalence of microalbuminuria among men by 52% and among Blacks by 26%. Adjustment of A/C permitted more accurate estimation of albumin excretion rate. Men and Blacks have a higher albumin excretion rate than do women and Whites and may thereby have an increased risk of microvascular and macrovascular disease.