To determine the feasibility of accurately assessing proteinuria using random urine samples, 24-hour urine samples were obtained from five healthy adult volunteers and 15 children (ages 5 to 17 years) with renal disease. Protein concentration was measured using a Coomassie blue binding technique, and total protein excretion was compared with the urinary protein/creatinine ratio (Up/Ucr). Total protein excretion varied from 28.3 mg to 12.3 gm, or 15.4 to 8500 mg/m2/day, and Up/Ucr ranged from 27.9 to 7650 micrograms/mg in the 24-hour samples; the correlation was highly significant (r = 0.99; P less than 0.001). Reference values for urinary protein concentration and Up/Ucr were established by examining random urine samples in 219 healthy individuals (ages 1 month to 61 years). Age-related differences in protein concentration were noted, and 95th percentiles were calculated as the upper limit of normal. The results of our study indicate that Up/Ucr is an accurate assessment of quantitative protein excretion and could largely replace the collection of timed urine samples for the assessment of proteinuria.