We describe a case study of comorbid obsessive compulsive disorder (OCD) in a nine-year-old boy with Duchenne muscular dystrophy (DMD). Patient history included persistent deficits in social communication and restrictive and repetitive patterns of behavior: a diagnosis of autism spectrum was formalized. Due to serious disruption on social functioning and negative development of the obsessive behavior we decided to start pharmacotherapy. Fluoxetine 5 mg/day was started and gradually increased to 20 mg/day. A significant positive effect was observed by both parents and teacher in daily functioning. Although parents reported a positive change in mood, formal behavior rating by them did not reveal a significant effect, reflecting the insensitivity of general behavior rating scales. However, neuropsychological testing revealed a significant effect. This case report highlights the diagnostic and therapeutic challenges of complex neuropsychiatric comorbidities in DMD. It is the first scientific report on fluoxetine effectiveness in this patient group. Further research is needed.