Up to two thirds of adults with intellectual disability have a comorbid psychiatric disorder, including mood disorders, autism, ADHD, schizophrenia and conduct disorder. The more severe is the disability, the higher the rate of psychiatric disorders. The presentation of medical problems can mimic psychiatric signs and symptoms and thus hinder proper diagnosis and treatment. Medical causes can include: medication side effects (such as antipsychotic medications causing dystonia or akathisia), metabolic deficits (such as diabetes and vitamin deficiencies), sleep disorders (such as sleep apnea syndrome), and many more. Medical problems may be masked by language and communication thus not addressed by the general practitioner. A psychiatrist specializing in adults afflicted by intellectual disability should be aware of possible physical diseases that might mimic psychiatric disorders to allow for better medical care.