Our report demonstrates anti-ARHGAP26 autoantibodies in a male patient with significant Lewy body pathology based on two core clinical features (fluctuating cognitive deficits with deficits in attention, and Parkinsonism) and an indicative biomarker (reduced dopamine transporter uptake) according to the McKeith criteria (McKeith et al., 2017). The gene discussed is ARHGAP26; the disease is Parkinson disease.