The epidemiology of myasthenia gravis (MG) was studied in the province of Trento (northern Italy) among 444,879 inhabitants. On prevalence day (December 31, 1990) the crude rate was 82.9 per million population: 113 for females and 50.7 for males. The incidence rate was 7.4 per million per year: 10.8 for females and 4.6 for males. Over the same observation period (1981-1990), we calculated a death rate of 1.12 per million population per year. MG predominates in females. Nevertheless, we observed some differences related to age and to presence of thymoma, and our data supports the recognition of three types of MG: (1) predominating in females with a peak frequency at about 20-29 years; (2) affecting both males and females, with a frequency increasing with age, and (3) associated with thymoma.