•Rhythmic delta is increasingly seen in anti-NMDA receptor encephalitis (NMDARE) refractory to first line immunotherapy.•Rhythmic delta decreases after second line immunotherapy and predates clinical improvement.•Rhythmic delta is not seen in other autoimmune encephalitides and is likely specific to NMDARE. Anti-NMDA receptor encephalitis (NMDARE) may not respond to first line immunotherapy. Biomarkers to track disease course and guide escalation of immunotherapy are needed. We describe the evolution of EEG in four patients with NMDARE requiring prolonged intensive care. Within a database of 121 patients with immune-mediated neurological disorders, ten with NMDARE were retrospectively identified. Four patients did not respond to first line immunotherapy. Continuous EEG was reviewed and correlated with clinical status and treatment. Intermittent polymorphic delta slowing was present in all patients. Generalized rhythmic delta occupied increasing proportion of the EEG as disease progressed, at times with superimposed beta. The institution of second line immunotherapy was followed by progressive decrease in rhythmic delta, predating clinical improvement. In one patient who did not respond to second line immunotherapy, rhythmic delta continued to occupy a majority of the recording. The extreme delta pattern was not seen in a comparison cohort of patients with autoimmune encephalitis without anti-NMDA-R antibodies. Extreme delta, with or without brushes, increases with progression of NMDARE, responds to escalation of immunotherapy, predating clinical improvement, and is likely specific to NMDA-R antibodies. Extreme delta may be a surrogate marker of disease activity in NMDARE refractory to first line immunotherapy.