Introduction The histopathologic changes of skeletal muscle in patients with myasthenia gravis have been well documented in many large autopsy series. More recently, intravital staining and electron microscopy have demonstrated tissue alterations in the presynaptic portion of the neuromuscular junction. The present study is designed to evaluate the usefulness of the muscle biopsy as a diagnostic aid in myasthenia gravis. In addition, the significance of some histological changes previously described will be reconsidered and discussed. Methods Muscle specimens from 37 patients with myasthenia gravis were examined in the study. The criteria for diagnosis were an appropriate history of muscle weakness and fatigue related to exercise and a response to anticholinesterase medication. No second disorder was present in any of these patients that might affect skeletal muscle. Specimens from patients with myasthenic syndromes secondary to carcinoma of the lung or thyrotoxicosis were excluded. Tables 1-3 summarize clinical information pertinent to the