High rates of aggressive behaviors and severe mood disturbances are documented in children with autism spectrum disorders (ASD) and limited literature documented the presence of bipolar disorder (BPD) comorbidity in ASD populations. Besides, there is a high incidence of BPD in family members of children with ASD. Munesue et al. speculated that the major comorbid mood disorder in individuals with high functioning ASD is BPD and referred that both disorders may share certain common vulnerability genes. Here we report an adolescent case with autism, which later diagnosed as having bipolar disorder in our clinic and discuss the clinical characteristics and bipolar symptomatology in this population. A 14-year-old boy was brought to our clinic by his parents with complaints of expansiveness, laughing excessively, singing constantly and insomnia. His symptoms had started two weeks before their referral. His parents’ first admission to a psychiatrist was at the age of nine with complaints of aggressiveness, self-injury, lack of eye contact, delay in language development and repetitive behaviors and he was diagnosed with autism. Although there was a considerable time lag between his parents’ recognition of the symptoms and their first referral; his autistic features were remarkable. The eye contact was poor, his spoken language was restricted and he had echolalia. He did not show interest in social interaction and had stereotypic hand movements. His medical history revealed epilepsy and he was using 1000 mg/day sodium valproate. His family history was insignificant except having a brother with epilepsy. About three months ago, he was seen by another psychiatrist for symptoms including irritability, being withdrawn and crying spells. He was diagnosed with depression and 2 mg/day risperidone and 20 mg/day şuoxetine were prescribed. Eight weeks later, above-mentioned symptoms started. His mood was elevated and irritable, his psychomotor activity was increased and he had insomnia. A manic shift was suspected. Fluoxetine treatment was ceased and risperidone was increased up to 3 mg. In the mean time, he was receiving 1500 mg/day sodium valproate for his seizures. The manic symptoms were successfully treated with 3 mg/day risperidone and 1500 mg/day sodium valproate combination in a few weeks but he had another manic episode two months later. A diagnosis of bipolar disorder was made and he had four mood episodes within the following 12 months period. The patient is still being monitored for ASD and comorbid rapid cycling bipolar disorder. The actual incidence of BPD in youth with ASD is probably underestimated. BPD should be borne in mind if a child with ASD refers with episodic irritability and aggressive attacks and have a family history of BPD.