A novel relationship between pancreatico-biliary secretion and autistic syndrome is disclosed. This relationship enables a novel therapy for the treatment of the symptoms of autistic syndromes, comprising the administration of a therapeutically effective, preferably intravenous, dose of secretin to an individual with autistic syndrome. The relationship further enables a differential diagnosis for autistic syndrome, comprising an analysis of an individual's blood and/or intestinal tissue for the presence of secretin and comparison of the level of secretin to known norms.