We retrospectively evaluated the effect of plasmapheresis (PE) in seven patients with paraneoplastic encephalomyelitis (PEM), small–cell lung carcinoma, and anti–Hu antibodies, and four patients with paraneoplastic cerebellar degeneration (PCD), ovarian or breast cancer, and anti–Yo antibodies. In addition to PE, patients received prednisone (nine), cyclophosphamide (eight), or treatment of the tumor (five). All but one patient were severely disabled by the time PE began. The clinical outcome was compared with that of five patients (PEM, four; PCD, one) who only had treatment of the tumor. Only one of these five patients had a severe neurologic deficit at the onset of the antineoplastic treatment. No patient improved. Two patients treated with PE and antineoplastic therapy and three who only received treatment of the tumor remained stable for at least 6 months. Four of the five patients with a stable course started the treatment when the neurologic deficit was not severe. We conclude that the efficacy of PE with other immunosuppressive therapies in the stabilization of the neurologic deficit is uncertain. © 1992 American Academy of Neurology.