Two cases of a neuromuscular hyperactivity syndrome associated with a proliferative thymoma and high serum titres of acetylcholine receptor (AChR) antibody with no signs of myasthenia are reported. The clinical and electrodiagnostic findings indicated generalized cholinergic hyperactivity at the neuromuscular junction and in the autonomic and central nervous system, resulting in generalized myokymia, excessive sweating and intermittent psychotic behaviour. The association with thymoma and raised AChR antibody suggests that this syndrome represents a unique type of autoimmune disease, in which antibodies against the AChR facilitate rather than inhibit cholinergic action. This conclusion is supported by the remission of symptoms after thymectomy and with immunosuppressive therapy in one case.