Many forms of rapidly progressive cognitive impairment are irreversible and rarely treatable. We present a case of autoimmune limbic encephalitis associated with voltage-gated potassium channel autoantibodies that led to rapid cognitive deterioration and bizarre behaviour, which subsequently improved following immunosuppressant therapy. The diagnosis of autoimmune limbic encephalitis needs to be considered in patients presenting with apparent viral encephalitis, in more unusual clinical situations in subjects with acute psychiatric illness and in cases of refractory epilepsy, particularly if there is evidence of amnesia. The diagnosis depends on the detection of specific antibodies directed against central nervous system targets. There can be a good response to immunosuppressant therapy.