The clinical and biochemical data on 13 patients with Batten's syndrome are described. Clinically the disease was characterized by progressive maental and somatic deterioration. Initially, vision loss was found between the ages 4 and 8 years. This was associated with 1 or 2 years of normal school attendance followed by attendance at a school for mentally retarded from the age of 8 to 11; then warding was established at a school for blind children and later on a hospital for epileptic patients when seizures and mental retardation made hospitalization necessary. Biochemically, an increased peroxidation rate was revealed in peripheral thrombocytes. This abnormality was associated with a significant decrease in peroxidase activity of leucocytes assayable with p-phenylenediamine, but not with Guajacol. The peroxidase defect seemed to concern an azide-resistant peroxidase. However, in serum the glutathione peroxidase was only found insignificantly decreased.