http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051515 Background Numerous associations between brain-reactive antibodies and neurological or psychiatric symptoms have been proposed. Serum autoantibody against the muscarinic cholinergic receptor (mAChR) was increased in some patients with chronic fatigue syndrome (CFS) or psychiatric disease. We examined whether serum autoantibody against mAChR affected the central cholinergic system by measuring brain mAChR binding and acetylcholinesterase activity using positron emission tomography (PET) in CFS patients with positive [CFS(+)] and negative [CFS(−)] autoantibodies. Methodology Five CFS(+) and six CFS(−) patients, as well as 11 normal control subjects underwent a series of PET measurements with N-[11C]methyl-3-piperidyl benzilate [11C](+)3-MPB for the mAChR binding and N-[11C]methyl-4-piperidyl acetate [11C]MP4A for acetylcholinesterase activity. Cognitive function of all subjects was assessed by neuropsychological tests. Although the brain [11C](+)3-MPB binding in CFS(−) patients did not differ from normal controls, CFS(+) patients showed significantly lower [11C](+)3-MPB binding than CFS(−) patients and normal controls. In contrast, the [11C]MP4A index showed no significant differences among these three groups. Neuropsychological measures were similar among groups. Conclusion The present results demonstrate that serum autoantibody against the mAChR can affect the brain mAChR without altering acetylcholinesterase activity and cognitive functions in CFS patients.