Dandy-Walker syndrome is a congenital brain malformation. The major presentations are mental disorder, cerebellar ataxia and symptoms related to hydrocephaly. It can be associated with psychiatric symptoms. Although, since this presentation is less prevalent, it has been insufficiently discussed

The subject is a 17 year old girl suffering from mental retardation. Her signs started 3 months ago when she was considered as having decreased need to sleep and walking during the night mostly. Her appetite increased as well. she was found to be getting more talkative than past; especially she talked to herself more.

She was admitted to hospital with the diagnosis of bipolar I disorder (the most recent episode: mania) in axis I and deep MR in axis II. In her brain MRI, enlargement of fourth ventricle and cisterna magna and atrophy cerebellar vermis were seen. EEG showed unspecific changes and there was no positive finding in her blood count, electrolytes,renal and liver function tests and blood sugar.

Despite two weeks of treatment with risperidone 3 mg/day and biperidine 4 mg/day and partial control of some symptoms, irritability and anger remained consistently. So, valproic acid (VPA) 400 mg/ day was added to treatment and after one month the patient was discharged from hospital with complete remission of newlypresented symptoms.

This report indicates the importance of taking psychiatric symptoms into consideration while evaluating organic brain disorders. With respect to treatment, the same treatment as primary mood disorders was indicated effective.