Regular non-medical treatment of a 6-year-old female patient with autism spectrum disorder failed due to comorbid compulsions and hallucinations. Differential diagnosis included obsessive-compulsive disorder and psychosis. The patient’s young age complicated accurate diagnosis and management. In this case we opted for a diagnostic follow-up, resulting in treatment with a selective serotonin reuptake inhibitor because of the patient’s frequent compulsions. This reduced the symptoms significantly.