Neuromyotonia is a syndrome of spontaneously occurring muscle activity of peripheral nerve origin, which can be triggered by voluntary or induced muscle contraction. It is one among several causes of visible myokymia. Although neuromyotonia may sometimes accompany hereditary neuropathies, most cases are acquired. The abnormal activity is characterized electromyographically by doublet, triplet or multiplet single unit discharges that have a high intraburst frequency, the frequency of the bursts themselves being irregular. Fibrillation potentials and fasciculations are often also present. We report five patients meeting these criteria. Clinical evidence suggesting a possible autoimmune aetiology included the presence of oligoclonal bands in the spinal fluid of the three patients examined and improvement following plasma exchange in two of three patients treated. Other supporting evidence from an English language literature review of ˜40 patients reported in the last 20 years includes an associated thymoma in five cases, myasthenia gravis (two cases), raised anti-acetylcholine receptor antibody titres (two cases) and induction by penicillamine (one case). Finally, two further patients have recently been reported to benefit from plasma exchange. These clinical data, taken together with the physiological changes observed in mice injected with patients' immunoglobulin G (reported separately), suggest that antibody-mediated autoimmune mechanisms, possibly directed to peripheral nerve K⁺ channels, may be important in the aetiology of acquired neuromyotonia.