Anti-N-methyl-D-aspartate receptor encephalitis is a severe, potentially treatable, disorder and prognosis depends on early recognition and prompt immunotherapy. We report a case of anti-N-methyl-D-aspartate receptor encephalitis with atypical age and gender, and a characteristic electroencephalographic pattern that supported the diagnosis. A 66-year-old male presented with psychiatric disturbances and focal seizures with alteration of consciousness, and progressed to a state of akinetic mutism. Auxiliary tests were negative or non-specific for anti-NMDAR encephalitis. Electroencephalographic monitoring revealed a unique pattern; the extreme delta brush. The patient improved with immunotherapy and was asymptomatic at six months of follow-up. Ancillary testing was positive for anti-N-methyl-D-aspartate receptor antibodies. Extreme delta brush is a recently described electroencephalographic pattern presenting in only one third of patients with anti-N-methyl-D-aspartate receptor encephalitis. The identification of this pattern, as in our case, may guide early diagnosis and treatment of anti-N-methyl-D-aspartate receptor encephalitis.