Demented people frequently show behavioural and psychiatric symptoms, including agitation, aggression, depression, anxiety, irritability, disinhibition, hallucination and delusion, which are collectively referred to as behavioural and psychological symptoms of dementia (BPSD). In some cases, these individuals be admitted to a psychiatric hospital because of the appearance or exacerbation of BPSD. However, even after admission, BPSD affect their quality of life. It is therefore important to clarify the pathophysiology of BPSD and to develop an effective treatment for them. Although the causes of BPSD are considered to be multiple, including biological, psychological, and environmental variables, the involvement of pathological changes in diseased brains is still not clear. Here, we report a case of concomitant argyrophilic grain disease (AGD) and dementia with Lewy bodies (DLB) who required admission to a psychiatric hospital because of severe anxiety, agitation, and depression. We then discuss the association between the distribution of pathological changes and the appearance of such psychiatric symptoms. An 81-year-old woman complained of chest discomfort, a pounding heart, the feeling of being smothered, and anxiety without any physical findings. She was referred to a psychiatrist at the age of 83 and diagnosed with having panic disorder. Paroxetine was prescribed, but her symptoms remained. She often got agitated and complained of suicidal thoughts. Because the cognitive decline progressed gradually, she was referred to another psychiatrist the same year. At the first visit, she complained of agitation, palpitation, and dyspnoea but denied