We examined the relative feasibility of using random urinary dipstick testing and urinary protein/creatinine ratios in the quantitation of proteinuria. Sixty-four children with relapsing nephrotic syndrome, ranging in age from 1 1/2 to 16 years, contributed 145 timed urine collections and 150 random specimens, which were analyzed by urinary protein dipstick, quantitation of protein and creatinine, or both. Total protein excretion was expressed as grams per surface area per day and the urinary protein/creatinine ratio as milligrams of protein per milligram of creatinine. Degrees of proteinuria were designated as physiologic (less than 0.1 gm/m2/day), intermediate (greater than 0.1 and less than 1.0 gm/m2/day), or nephrotic (greater than 1.0 gm/m2/day). The log regression analysis of the 24-hour urinary protein/creatinine ratio (y) and total protein excretion (x) was highly significant (r = 0.97; p less than 0.001). The upper and lower confidence limits of the urinary protein/creatinine ratio (1.0 and 0.1, respectively) closely approximated the designations for nephrotic and physiologic proteinuria, respectively. These values were therefore used to classify degrees of proteinuria by the urine protein/creatinine ratio. The validity of these tests was assessed by sensitivity, specificity, and predictive value, and compared with that of random testing by urinary dipstick. The urinary protein/creatinine ratio offered good reliability as a test for classifying degrees of proteinuria and accurately predicting nephrotic and physiologic proteinuria. The random dipstick testing was reliable only when results were distinctly positive and when sensitivity and specificity were low. The error in the total protein excretion value that was imposed by collection errors with high and low variations in the creatinine value (57% of samples) was largely corrected by normalization of the data by log transformation. When controlled for creatinine excretion, linear regression analysis allowed calculation of total protein excretion (TP) from the urinary protein/creatinine ratio (U P/Cr) by the following equation: TP (gm/m2/day) = 0.63 (U P/Cr) at all levels of proteinuria. The random urinary protein/creatinine ratio appeared to offer the most sensitive test for classification of proteinuria in children with nephrosis, with the advantages of ease and expediency not afforded by the 24-hour urine quantitation.