Objectives: Arachnoid cysts are benign congenital malformations of the arachnoid. Many anecdotal reports as well as a few systematic studies indicate an association of various psychiatric symptoms and several aspects of cognitive problems in arachnoid cyst patients. Here we report to the best of our knowledge the first coincidence of bipolar disorder and the finding of an arachnoid cyst and discuss the clinical implications. Methods and results: The case of a patient with a long term history of bipolar disorder and cognitive impairment and the incidental finding of a large arachnoid cyst with adjacent chronic subdural haematoma is described and related to results of a literature review. Conclusions: There might be a causal relationship between the existence of an arachnoid cyst and the symptoms of bipolar disorders. Neurosurgical treatment to improve cognitive impairment and psychiatric symptoms should be discussed carefully. Further functional studies to allow evidence based treatment recommendations are necessary.