A simple method is described for determining the uric acid: creatinine ratio (UA/C ratio) in urine collected on filter paper. Uric acid and creatinine can be eluted quantitatively from Schleicher and Schuell 903 filter paper in alkaline buffer; the eluate is analyzed for the two compounds simultaneously by an automated method. The mean UA/C ratio was defined in 820 newborn infants in the postnatal period of life. After the third day, the average is 1.49 ± 0.60 (mean ± 1SD), falling from a high value of 2.25 ± 0.85 on the second day. There is no significant sex difference or diurnal variation at this age and neither gestational age nor birth weight apparently affects the ratio. The technique can be used to screen population, particularly the neonate after the 3rd day of life, for disorders of purine metabolism. An abnormal ratio (i.e., above 3.29 or > 3SD above the mean) would indicate such a disorder. Transient high UA/C ratios can be anticipated in at least 0.5% of samples often because of acquired perinatal complications.