We have evaluated a tandem mass spectrometry method for determining free carnitine concentrations in plasma and have compared its performance with that of an existing radioenzymatic assay. In this method, plasma was mixed with an internal standard, carnitine-d3 (600 nmol/L), and butylated before analysis on a Quattro II tandem mass spectrometer. The detection limit of the tandem mass spectrometric (MS) method was 4 µmol/L and carryover between samples was 2·2%. Precision of the method was 1·6-5·4% between injections and 4·0-5·6% between samples. Between-batch precision was 10-13%. The method was linear up to a free carnitine concentration of 300 µmol/L and good agreement was found with the existing radioenzymatic assay. We conclude that the tandem MS method is a precise and robust method for the determination of free carnitine concentrations in plasma that overcomes the disadvantages of the radiochemical method.