Population-based studies of the epidemiology of myasthenia gravis (MG) have been conducted for over 50 years. Over that time, there has been a clear trend towards an increase in the reported prevalence of the disease. In recent years, there has also been an interest in determining a reasonably accurate estimate of the number of MG patients in the United States. Current estimates place the prevalence at a high value of about 20 per 100,000. The year 2000 U.S. population estimate is slightly less than 280 million. A crude estimate of the number of MG patients derived from the population estimate and the reported prevalence from selected populations indicates that there are between 53,000 and 59,900 patients in the United States at this time. The age and ethnic distributions in the United States are evolving from those that were present when the majority of the population-based studies were done, and the distribution of severity of the disease may also have changed. Future studies of the epidemiology of MG should take these factors into account, and further research into the economic and quality-of-life impact of the disease in the population is needed.