We describe a 56-year-old woman with hyperammonemic encephalopathy caused by side effect of valproic acid (VPA). Ten months before the admission to our hospital, she had the first attack of convulsive seizure. Diagnosis of epilepsy was made, and the oral administration of VPA (800mg/day) was started at another hospital. Seizure was well controlled by VPA, until the recurrence of attack forced her to visit our hospital. Convulsive seizure disappeared immediately after intravenous administration of diazepam, but consciousness disturbance was prolonged for a few days. Since laboratory examinations revealed hyperammonemia without liver dysfunction, VPA was discontinued. Subsequently, her consciousness completely recovered. Other laboratory findings, including positive antinuclear antibody, antibodies to Sjögren's syndrome A, reduced lacrimal secretion in Schirmer's test, and cell infiltration in the salivary gland on lip biopsy specimen, suggested the presence of Sjögren's syndrome. The hyperammonemia occurs by the side effect of VPA, often has basal disease or drug interactions. It was, however, especially in patients with basal disease or other drugs, obscure whether (and how) Sjögren's syndrome contributed to the development of hyperammonemic encephalopathy in this case, since she took only VPA.