An abnormality in the pyridoxal-5'-phosphate (PLP) dependent enzyme, glutamic acid decarboxylase (GAD), which synthesizes gamma-aminobutyric acid (GABA), may underlie the epileptic syndrome of pyridoxine-dependent seizures. GABA synthesis by skin fibroblasts from an infant with pyridoxine-dependent seizures, and from five controls, was measured. PLP independent GAD activity was similar in control and patient fibroblasts, whereas the patient's PLP dependent GAD activity was reduced compared with controls. These findings support the hypothesis for the expression of this familial disease.