The California Encephalitis Project (CEP), established in 1998 to explore encephalitic etiologies, has identified patients with N-methyl-D-aspartate receptor (NMDAR) antibodies, the likely etiology of their encephalitis. This study compares the presentation of such patients to those with viral encephalitis, so that infectious disease clinicians may identify individuals with this treatable disorder. Patients were physician-referred, and standardized forms were used to gather demographic, clinical, and laboratory data. Features of anti-NMDAR+ patients were compared with the viral encephalitides of enteroviral (EV), rabies, and herpes simplex-1 (HSV-1) origins. Sixteen cases with confirmed viral etiologies were all negative on NMDAR antibody testing. Ten anti-NMDAR+ patients were profiled with a median age of 18.5 years (range 11-31 years). None were Caucasian. They had a characteristic progression with prominent psychiatric symptoms, autonomic instability, significant neurologic abnormalities, and seizures. Two had a teratoma, and, of the remaining eight, four had serologic evidence of acute Mycoplasma infection. The clinical and imaging features of anti-NMDAR+ patients served to differentiate this autoimmune disorder from HSV-1, EV, and rabies. Unlike classic paraneoplastic encephalitis, anti-NMDAR encephalitis affects younger patients and is often treatable. The association of NMDAR antibodies in patients with possible Mycoplasma pneumoniae infection warrants further study.