Measurements of protein-to-creatinine ratios in single-voided urine samples were compared with 24-hour urinary protein excretions for quantitation of proteinuria in inpatients and outpatients. Patients included those representing a broad spectrum of renal diseases, a wide range of proteinuria, and various degrees of reduction in glomerular filtration rate. Protein-to-creatinine ratios in single-voided urine samples correlated well with measurements of 24-hour urinary protein. This simple single-voided test is reliable and useful in the screening, assessment, and follow-up of proteinuria and avoids the problems associated with 24-hour urine collection.