Another patient with the rare convulsive disorder termed "vitamin B6 (or pyridoxine) dependency" is presented. The case of a deceased sibling, suspected of having had the same disorder, is also presented. The characteristic features of the syndrome, as illustrated in these patients, are discussed. The unusual daily requirement for 10 or 11 mg of pyridoxine supplementation has persisted during the first 20 months of the surviving child's life. The response to pyridoxine is specific and impressive, suggesting a biochemical lesion underlying the convulsive disorder. The probable nature of this biochemical disorder, the significance of a peculiar pattern of urinary excretion of vitamin B6 and related products, and the sequence of vitamin B6 dependent enzyme involvement, are discussed; the disease may be an inborn error of metabolism involving an enzyme of the brain. Nothing can be lost, and perhaps much will be gained, if a diagnostic trial with pyridoxine therapy produces impressive improvement in any "idiopathic" convulsive disorder of infancy and childhood.