The behavioral variant of frontotemporal dementia is usually a sporadic and progressive neurodegenerative disorder. Here, we report the subacute onset of a frontotemporal dementia phenotype with a treatable etiology. The patient has a history of rheumatoid arthritis, episcleritis, and thyroid eye disease on immunosuppressive therapy. He experienced a rapid personality change, including inappropriate behavior, which suggested frontotemporal dementia. Results of imaging and neuropsychological testing also suggested frontotemporal dementia. Because of his autoimmune diseases and unusually short onset of symptoms, serum paraneoplastic panel and cerebrospinal fluid were analyzed and revealed elevated P/Q- and N-type calcium channel antibodies. Treatment with therapeutic plasma exchange resulted in a rapid improvement of his behavior and cognition. This case suggests that there may be some treatable causes of frontotemporal dementia symptomatology, that is, paraneoplastic antibodies. In the context of atypical features of frontotemporal dementia, practitioners should maintain a high index of suspicion.