A patient with an intermittent movement disorder has been found to have an inherited defect in pyruvate decarboxylase ((2-oxo-acid carboxy-lyase, E.C. 184.108.40.206.). The patient is a 9 yr old boy who since infancy has had repeated episodes of a combined cerebellar and choreoathetoid movement disorder. He has an elevated level of pyruvic acid in his blood, an elevated urinary alanine content, and less marked elevations in blood alanine and lactate. Methods were developed to study his metabolic abnormality in dilute suspensions of white blood cells and cultured skin fibroblasts, as well as in cell-free sonicates of fibroblasts. Oxidation of pyruvic acid-1-(14)C and pyruvic acid-2-(14)C by his cells and pyruvate decarboxylase activity in sonicates of his cells were less than 20% of those in cells from control subjects. Oxidation of glutamic acid-U-(14)C, acetate-1-(14)C, and palmitate-1-(14)C was normal, as was incorporation of alanine-U-(14)C into protein. The rate of oxidation of pyruvic acid by the father's cells and the activity of pyruvate decarboxylase in the father's sonicated fibroblasts were intermediate between those of the patient and those of controls. Values for the mother were at or just below the lower limits of the ranges in controls. Kinetic data suggested the posibility of several forms of pyruvate decarboxylase in this family. Possible mechanisms relating the chemical abnormality and the clinical symptoms in this patient are discussed.