A case report is presented of a boy with the infantile spasm syndrome beginning at eight months of age. He had a clinical course marked by increasingly severe seizures and neurological regression. After death at twenty-one months of age, autopsy of the central nervous system revealed demyelination of white matter with sparing of arcuate fibers. An earlier born male sibling had had a similar clinical pattern but died without an autopsy. During his lifetime, the patient had markedly elevated levels of 5-hydroxyindoles (a measure of serotonin) in his blood. At autopsy, the level of 5-hydroxytryptamine (serotonin) in four grey matter areas of the brain was lower than those of a control who died on the same day. This is the first case reporting a comparison of blood and central nervous system levels of 5-hydroxytrypamine in a child.