OBJECTIVE To examine myasthenia gravis (MG) severity and long-term prognosis in seronegative, seropositive, and thymoma MG. METHODS Four series of patients were studied retrospectively. Severity and treatment were assessed each year, and muscle antibodies were assayed. RESULTS Seropositive MG patients had a more severe course than seronegative MG patients. MG severity was higher in non-thymectomized compared to thymectomized early-onset MG patients. MG severity did not differ between thymectomized and non-thymectomized late-onset patients. There was no significant difference in MG severity between thymoma and non-thymoma MG patients. CONCLUSIONS MG is more severe in seropositive MG patients. With proper treatment, especially early thymectomy, the long-term prognosis is good in seropositive MG patients. The present studies indicate a benefit of thymectomy in early-onset MG, but no dramatic benefit in late-onset MG. Similar MG severity and outcome was seen in thymoma and non-thymoma MG.