There is still uncertainty regarding risk factors for cancer occurrence in patients with myasthenia gravis (MG). The objective of this study is to determine the prevalence of extrathymic neoplasms in patients with MG and the factors associated with tumor occurrence. The archives of four tertiary MG centers were consulted and patients were interviewed on the main clinical features of the disease, the presence and type(s) of extrathymic neoplasms and other autoimmune disorders, and their symptomatic and immunosuppressant treatments (with detailed schedules). A retrospective cohort survey was undertaken comparing the demographic and clinical variables of patients with extrathymic neoplasms to those of the remaining MG population. 2,479 patients were traced and interviewed personally or through informants. The sample included 1,490 women and 989 men (mean age 54.7 years). Other autoimmune disorders were present in 216 cases (8.7%). Thymectomy was performed in 1,549 cases (62.5%), thymic hyperplasia and thymoma being the most common findings. Acetylcholinesterase-inhibitors were the most common treatment (93.5%), followed by steroids (64.3%), azathioprine (35.0%), plasma exchange (13.2%), immunoglobulins (7.5%), cyclosporine (5.3%), and cyclophosphamide (5.0%). 221 patients (8.9%) had one or more extrathymic tumors, 168 of which occurred after disease onset. Patients with and without extrathymic neoplasms were followed for 14.8 and 13.9 years, respectively. Variables shown by multivariate analysis to be associated with increased neoplastic risk included older age, thymoma and immunoglobulin use. Extrathymic tumors are a common finding in patients with MG and tend to be associated with age, thymoma, and immunoglobulin use.