To assess the suitability of lymphocytes for patient diagnosis and carrier detection of lipoamide dehydrogenase deficiency, the activity of lipoamide dehydrogenase was determined in lymphocytes of six patients, seven obligate heterozygotes and 32 healthy controls. In healthy controls, lipoamide dehydrogenase activity was 80.7 +/- 23.6 nmol/min/mg protein, in obligate heterozygotes it was 36.0 +/- 12.1 nmol/min/mg protein and in the patients it was 13.3 +/- 5.1 nmol/min/mg protein. For the purpose of standardization, the results were also calculated as lipoamide dehydrogenase/citrate synthase activity ratio. The activity of lipoamide dehydrogenase and the lipoamide dehydrogenase/citrate synthase ratio differed significantly between the three groups (P < 0.005). We conclude that lymphocytes are suitable for the diagnosis of lipoamide dehydrogenase deficiency and for carrier detection.