A 5-year old boy with intermittent sopor, severe vomiting and ataxia since 2 1/2 years old was described. During attack period the patient showed elevation of blood pyruvate and alanine and also cerebrospinal-fluid pyruvate, whereas during the time free from attack only blood pyruvate was increased. Deficiency of pyruvate decarboxylase was found in patient's leukocyte and in cultured skin fibroblasts. Addition of thiamine in assay medium did not correct the enzyme activity. Oral administration of citrate seemed to be most effective and during the treatment only a mild attack was once observed.