A high-performance liquid chromatography-mass spectrometry method has been developed for the simultaneous determination of native carnitine and eight acylcarnitines in urine. The procedure uses a solid-phase extraction on a cation-exchange column and the separation is performed without derivatization within 17 min on a reversed-phase C8 column in the presence of a volatile ion-pairing reagent. The detector was an ion trap mass spectrometer and quantification was carried out in the MS-MS mode. Validation was done for aqueous standards at ranges between 0.75 and 200 micromol/l, depending on the compound. Carnitine was quantified in urine and comparison with a radioenzymatic assay gave a satisfactory correlation (R2 = 0.981). The assay could be successfully applied to the diagnostic of pathological acylcarnitines profile of metabolic disorders in urines of patients suffering from different organic acidurias.