Inborn errors of metabolism represent rare but important causes of psychiatric disorders in adoles- cents or adults. The mucopolysaccharidoses (MPSs) are a group of seven inherited metabolic disorders within the larger lysosomal storage disease (LSD) family. Sanfilippo type B is an autosomal recessive mucopolysaccharidosis(MPSIIIB) caused by deficiencyof N-acetyl-a-D- glucosaminidase, a lysosomal enzyme involved in the degradation of heparan sulfate. The birth prevalence of the disease ranges from 0.28 to 4.1 per 100,000. Affected subjects show developmental delay, attention deficit disorder, uncontrollable hyperactivity, and aggressive behaviour, followed by progressive dementia with death usually among late teens. A 15 years old mild mentally retarded adolescent with a diagnosis of Sanşippo type 3B( MPS 3B) presented with behavioural problems to our outpatient clinic. His parents reported that he was consulted by child and adolescent psychiatry department to genetic diseases department due to macrocephaly, abnormal EEG findings, syndromic facial appearence, hyperactivity and aggresive behaviours that were resistant to methlyphenidate and thioridazine and then diagnosis of MPS 3B was made by urine analysis at the age of six. During our assessment, his parents noted uncontrollable crying, his concentration was poor, he had marked hyperactivity. He had been taking risperidone and chlorpromazine at that time. We first changed antipsychotic treatment to aripiprazole 5 mg once daily then increased dose up to 10 mg/day, which reduced his aggression and episodic cryings. Then we added atomoxetine 40 mg once daily, targeting hyperactivity and inattention and also in order to alleviate anxiety. One month after commencement of these medications there was a significant clinical improvement in his hyperactivity, inattention and aggressive behaviours. Our case is notable because conventional treatment, including behavioural modification and pharmacotherapy including stimulants and most of antipsychotics has achieved limited success managing these symptoms.