Dementia with Lewy bodies (DLB) is the second most common form of degenerative dementia after Alzheimer’s disease. In some patients with DLB, relatively rare delusions are known to emerge, such as Othello syndrome, delusional parasitosis and delusion of duplication. Erotomania, also known as de Clerambault’s syndrome, is characterized by the delusion that a person has fallen in love with the patient. It occasionally appears secondary to psychiatric disorders and organic brain diseases. However, there have been no reports on cases secondary to patients with DLB.

The patient was an 83-year-old woman who lived alone. Mild cognitive impairment appeared at the age of 82 years. Soon after, she had the delusional conviction that her family doctor was in love with her. Her symptoms, such as gradually progressive cognitive impairment, cognitive fluctuations, and parkinsonism, indicated DLB. Although small doses of quetiapine, brexpiprazole and risperidone were prescribed for the treatment of the delusion, each of them was discontinued soon because of the adverse reactions. Finally, the delusion was successfully treated with a small dose of blonanserin without sever side effects.

This case report suggests the possibility of de Clerambault’s syndrome during the early stages of DLB. Recently, psychiatric-onset DLB has increasingly gained attention in recent years. Further accumulation of knowledge about delusions in patients with DLB for an early diagnosis.