A 9-year-old girl with type I hyperprolinemia, who also had photogenic epilepsy, was reported. She showed epileptic discharges and the regression in speech and motor activities, since 7 years of age. Her plasma proline levels were 3 to 4 times higher than control levels. In urine, iminoglycinuria appeared, when plasma proline value exceeded 0.80 mM. The proline oxidase activity of the liver tissues obtained by biopsy in the patient was about 23.5%, compared to that of controls. In spite of the restriction of proline and protein intake, she showed progressive speech and motor retardation.