The aim of this study was to evaluate the severity of proteinuria using the protein/creatinine ratio in a random urine sample. In 45 patients (male 28, female 17; mean age 50.68 +/- 18.26 years) with proteinuria of various causes, we measured the 24-hour protein excretion per 1.73 m(2) of body surface and, during the same day, the protein/creatinine ratio in three different urine samples (8 am, 12 pm, 4 pm). The 24 h proteinuria was defined as mild (<1 g), moderate (1-3.4 g), and severe (>3.4 g) in 7, 27, and 11 patients, respectively. The sensitivity for protein/creatinine ratio compared to the 24 h proteinuria as a method of reference was 86-100% in the mild, 78-100% in the moderate, and 73-82% in the severe proteinuria, whereas the specificity was 84-100%, 78-83%, and 100% respectively. The patients with better renal function had significantly higher proteinuria levels. There was a similarity in the 24 h proteinuria and the protein/creatinine ratio measurements in all renal function and level-of-proteinuria groups. The protein/creatinine ratio of the morning and midday samples had a very good association with the 24 h sample, whereas it was not associated significantly with the evening sample (4 pm). In conclusion, the degree of 24 h proteinuria levels can be evaluated by calculating the protein/creatinine ratio in a random urine sample collected at any time from morning until midday. Protein/creatinine ratio is independent of the severity of proteinuria or renal function, and it can replace in clinical practice the cumbersome 24 h urine collections.