Patient: Male, 25-year-old Final Diagnosis: Autism spectrum disorder • celiac disease Symptoms: Behavioral disturbance • diarrhea • weigh loss Medication: — Clinical Procedure: — Specialty: Gastroenterology and Hepatology • General and Internal Medicine Objective: Rare co-existance of disease or pathology Background: Celiac disease is very common, with some estimates placing the prevalence at approximately 1: 300 worldwide. Typified by autoimmune degradation of the duodenal brush border due to reactivity with dietary gluten, causing malabsorption, it classically presents with both gastrointestinal and extra-intestinal symptoms. Gastrointestinal symptoms commonly include diarrhea, constipation, foul steatorrhea, flatulence, and bloating. With increased awareness and availability of testing, it is rare that a patient would present with celiac crisis, which is a syndrome of profuse diarrhea and severe metabolic/nutritional disturbances. In children, interestingly, celiac disease can present primarily as behavioral disturbance, such as increased aggression or anxiety, with milder or absent gastrointestinal symptoms. Case Report: A 25-year-old man with a history of schizophrenia and autism spectrum disorder presented for behavioral disturbance after breaking into a neighbor’s house to eat food. He also reported several months of diarrhea and fecal incontinence and was noted to have severe malnutrition on exam, despite dramatic food intake. Tissue transglutaminase IgA antibody (TTG) and gliadin IgA were highly suggestive of celiac disease, which was confirmed by biopsy. He was started on a lactose-free and gluten-free diet, and required a short course of total parenteral nutrition (TPN) for nutritional resuscitation. He improved rapidly with this intervention, and returned to nutritional and behavioral baseline. Conclusions: We report a unique case in which an adult with psychiatric comorbidities presented with predominantly behavioral disturbances, a more common presentation in children with the disorder. These patients may present in an atypical fashion, and the clinician should have a high index of suspicion.