Three different inherited disorders are known in which thiamine may exert a beneficial effect: maple syrup urine disease (MSUD), lactic acidaemia and the syndrome of megaloblastic anaemia with sensorineural deafness and diabetes mellitus. The amounts of thiamine which were used for long-term treatment varied from 20 to 2400 mg day-1. Additional treatment, such as the reduction of dietary branched chain amino acids in MSUD, could not be omitted in some cases. It has been shown that the vitamin improves the stability of the branched chain ketoacid decarboxylase, although some weeks may be needed to observe the in vivo effect of treatment. A prolonged trial with high doses of thiamine should always be given.