In this report we present a clinical case of trismus. The patient in question showed symptoms of trismus for 3 days, rapidly leading to respiratory insufficiency. Afterwards she developed myoclonus and progressive encephalopathy. Neurological workup showed no relevant abnormalities. A CT of the abdomen revealed a mass in the lower abdomen, which turned out to be an ovarian teratoma. Progressive encephalopathy with rigidity and myoclonus (PERM) was diagnosed clinically. Treatment with corticosteroids, benzodiazepines, and levetiracetam did not ameliorate the patient's condition. Only after the introduction of plasmapheresis was there a spectacular improvement in her clinical state. In this case we could not detect associated antibodies. The most likely cause of PERM is paraneoplastic disease secondary to ovarian teratoma. This type of tumor has been associated with multiple paraneoplastic neurological conditions, but this is the first case associated with PERM. To date there is only one publication on trismus as a sole presenting sign, with a quite similar disease course.