The role of cyclosporin A (CsA) in the treatment of resistant chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) was retrospectively reviewed in 19 patients who had failed to respond adequately to corticosteroids, plasmapheresis, intravenous immunoglobulin, and in some cases other immunosuppressive agents. Patients were subdivided into progressive or relapsing types according to the course of disease and response to therapy graded at follow-up by clinical and electrophysiological criteria. In the progressive group, the mean disability status declined from 3.8+/-0.7 to 1.8+/-1.1 grades on a 5-grade scale following CsA therapy (P<0.001). In the relapsing group, the mean annual incidence of relapse declined from 1.0+/-0.5 to 0.2+/-0.4 after commencement of CsA (P<0.05). Dose-dependent, reversible nephrotoxicity was the most serious complication of therapy, and necessitated cessation of CsA in 2 patients. In conclusion, CsA is an efficacious and, with appropriate monitoring, safe therapy for patients with CIDP.