A precise, simple and rapid method for the quantitative determination of primary amino acids in urine based on high-performance liquid chromatography and o-phthaldialdehyde pre-column derivatization is described. All primary urinary amino acids could be determined within 49 min (injection to injection). Amino acid concentrations in 40 urinary samples were measured by this method and the results were compared with those measured by ion-exchange chromatography. The correlation coefficient for the common amino acids was greater than 0.90. This is the first study in which such a detailed comparison has been made on urine samples. It appeared that the method described is an excellent alternative to the classical ion-exchange method for the quantitation of urinary amino acids.