Type I hyperprolinemia is an autosomal recessive disorder characterized by increased plasma and urine proline concentrations due to a deficiency of the enzyme, proline oxidase. This rare inborn error of proline metabolism is generally believed to be a benign condition although many associated clinical abnormalities have been reported. We report two siblings with Type I hyperprolinemia who presented with recurrent seizures. They had elevated plasma proline levels with massive prolinuria without an increased urinary excretion of delta 1-pyrolline-carboxylic acid.