A method is reported for determination of allantoin in urine and plasma based on high-performance liquid chromatography (HPLC) and pre-column derivatization. In the derivatization procedure, allantoin is converted to glyoxylic acid which forms a hydrazone with 2,4-dinitrophenylhydrazine. The hydrazone appears as syn and anti isomers at a constant ratio. These derivatives are separated by HPLC using a reversed-phase C18 column from hydrazones of other keto acids possibly present in urine and plasma and then monitored at 360 nm. All components were completely resolved in 15 min. Both the reagents and derivatization products are stable. Recovery of allantoin added to urine and plasma was 95 +/- 3.7% (n = 45) and 100 +/- 7.5% (n = 64), respectively. The lowest allantoin concentration that gave a reproducible integration was 5 mumol/l. The between-assay and within-day coefficients of variation were 2.8 and 0.6%, respectively.