The amount of protein excreted in the urine has diagnostic and prognostic importance and is used to assess the effectiveness of therapy. The method most commonly used to measure urinary protein relies on 24-hour urine collections, which are time consuming and often inaccurate because of collection errors. We reasoned that the urinary protein/creatinine ratio in a single voided urine sample should corre- late well with the quantity of protein in timed urine colletion. In a group of 69 patients with various renal diseases, we found an excellent correlation between the protein/creatinine in a single voided urine sample and the amout of 24- hour urine protein. The best correlation was found during the time of between 12: 00 noo n and 6: 00 p.m. (Y=p.57X+Q.75, Y; 24-hour protein excretion per body surface area, X; pro- tein/creatinine ratio in a single voided urine sam- ple during the time of between 12: 00 noon and 6: 00 p.m.). We conclude that the determination of the protein/creatinine ratio in a single voided urine during the time of between 12: 00 noon and 6: 00 p.m. can replace the 24-hour urine collection in the clinical quantitation of proteinuria.