We report a gas chromatographic assay for urinary medium-chain acylcarnitines which employs their property of thermal lability, and by circumventing the need for specialised mass spectroscopy is suitable for routine laboratory use. The method produces readily interpreted, uncomplicated chromatograms and has proved to be both sufficiently sensitive and specific to enable detection of octanoylcarnitine in a symptomatic individual with medium-chain CoA dehydrogenase deficiency and in two asymptomatic siblings following administration of a carnitine load.