A method based on the release of tritiated water from [9,10(n)-3H] palmitic and myristic acids previously described for fibroblasts, was adapted for lymphocytes for the rapid diagnosis of fatty acid oxidation disorders. Optimal concentrations for both substrates and linearity of the assay were established. Normal values were established in control subjects of different age groups (58 children and 117 adults) and 16 patients with known fatty acid oxidation disorders were tested. Tritiated water production from patients' lymphocytes was expressed as a ratio between residual oxidations of palmitate and myristate and the results show that this method allows good differentiation between long chain and medium chain fatty acid oxidation defects.