Purpose Alzheimer disease (AD) is an irreversible neurodegenerative disease that causes progressive cognitive decline. Co-existing thymoma should be considered when rapid deterioration of cognition was noted in AD patients and removal of thymoma may improve cognition in AD. Case report We report a 72-year-old woman with initial complaints of memory impairment for 2 years. After detailed history taking, neuropsychological tests, brain magnetic resonance imaging, and positive amyloid positron emission tomography, she was diagnosed as having dementia of the Alzheimer type. At the time of diagnosis, her dementia condition was mild (clinical dementia rating [CDR] is equal to 1, CDR sum of boxes [CDR-sb] = 4.5, Mini-Mental State Examination (MMSE) is equal to 21 divided by 30). She needed moderate assistance in performing daily life activities. One year after AD diagnosis, her condition deteriorated drastically, and she experienced frequent falls and severe weakness apart from cognitive symptoms. Concurrent myasthenia gravis (MG) with thymoma was found later, and thymectomy was performed. Her symptoms related to MG alleviated after the operation. Notably, her cognitive symptoms also improved 4 months after the operation, and her dementia reversed to mild cognitive impairment. Conclusion Although the role of neuroinflammation in AD has been widely discussed, it remains elusive. Removal of the co-existing thymoma not only alleviated the patient's MG symptoms but also improved her cognitive performance. We supposed that this effect may have been a direct result of the decrease in acetylcholine receptor antibody or reduction in the degree of neuroinflammation. Keywords Alzheimer disease, thymoma, neuroinflammation, central cholinergic effects, acetylcholine receptor antibody.