Epidemiology is the study of a disease in a population. The epidemiology of myasthenia gravis (MG) has been studied for over 50 years, and over 50 studies have been published in one form or another. The published prevalence has steadily increased, and one can estimate that there are approximately 60,000 patients with MG in the United States. The current trends indicate that, as the population ages, an increasing number of patients with MG can be expected, but the clinical patterns of the disease may change.