Proteinuria evaluation is important for the differential diagnosis of renal disease and the assessment of prognosis and response to therapy. Qualitative proteinuria correlates poorly with quantitative 24-hour protein levels because of hydration and methodologic problems associated with observer error. The authors automated quantitative urinary protein and osmolality measurements and evaluated whether a urinary protein-osmolality ratio (mg/L/mOsm/kg) in the random urinalysis predicts the 24-hour urine protein level. They compared this ratio with the (1) 24-hour protein levels in 78 normal women and 53 normal men; (2) 24-hour protein levels in 129 proteinuric patients; and (3) urinary protein-creatinine ratio in 131 normal patients and 105 proteinuric patients. The normal urinary protein-osmolality ratio was less than 0.12 (sensitivity, 96%; specificity, 93%). This compared to a urinary protein-creatinine ratio of less than 0.05 (sensitivity, 96%; specificity, 90%). A urinary protein-osmolality ratio of greater than 2.5 indicates a level of more than 3.0 g/24 hour (sensitivity, 91%; specificity, 98%). The urinary protein-osmolality ratio = 1.12x + 0.05 (r = 0.88), where x = 24-hour protein. The authors conclude that the quantitative urinary protein-osmolality ratio is better than a qualitative urinalysis and urinary protein-urinary creatinine ratio for detecting or assessing abnormal proteinuria and allows for a reasonable prediction of 24-hour protein levels.