During a comprehensive epidemiological study of myasthenia gravis (MG) in Western Denmark 1975-1989, we analyzed the occurrence, clinical characteristics and prognosis of associated autoimmune diseases (AAD) in MG patients. AAD were found in 20 of 212 incident cases (9%) and in 30 of 220 prevalent cases (14%). The most common diseases were: thyroid disorders and rheumatic arthritis. Clinically, it was not possible to identify a subgroup of MG patients with a higher risk of AAD. In most MG patients the AAD occurred before thymectomy. The severity of the AAD was not influenced by thymectomy. The remission rate was lower in MG patients with AAD than in MG patients without AAD suggesting that the autoimmune response in MG patients with AAD is more severe.