A relatively simple and specific method for measuring urinary orotic acid is reported. With this method, orotic acid is enzymatically converted to uridine 5-phosphate. The difference in absorbance produced by a simple colorimetric procedure in the presence and absence of the enzymatic conversion allows specific measurement of orotic acid. Normal values for urine are less than 4 μg/mg creatinine in infants and less than 3 μg/mg creatinine in older children and adults.