In patients with myasthenia gravis, vigorous exercise and electric stimulation produce weakness that is usually reversed by rest. We describe a case of persistent weakness in a 31-year-old woman with myasthenia gravis following therapeutic electric stimulation. She was injured in an automobile accident, with cervical and lumbar muscle strain diagnosed by a local emergency room physician. She consulted a chiropractor two days later because her symptoms had not abated. The chiropractic treatment of short-wave diathermy, high-voltage electric stimulation and spinal manipulation continued thrice weekly for six weeks. She then presented to her neurologist, complaining of persistent fatigue, weakness, increased diplopia, cervical and occipital pain, and disrupted sleep. Neck weakness had developed since her last visit. She was admitted to the hospital for evaluation. Neurologic exam showed no other abnormality of strength, sensation, or reflexes. The exacerbation of myasthenia responded to prednisone and increased pyridostigmine. After two months she required only her preinjury dose of pyridostigmine (60 mg q.d. p.r.n.) to prevent diplopia or fatigue, and her strength was normal. Repeated electrically induced muscle contraction is suspected of causing this exacerbation of myasthenia gravis.