A long term follow up study is presented of 73 patients with myasthenia gravis, living in Amsterdam between 1926 and 1965. In the period 1961-65 the annual incidence was 3.1, the prevalence 53 per million. Maximum severity of the disease occurred during the first seven years after onset in 87%. Eighteen (29%) patients died, of whom eight had a thymoma (TH). Spontaneous improvement or remission occurred at any time during the follow up. At the end of the study (1985) 16 (22%) patients were in a complete clinical remission, 13 (18%) had improved considerably (3 with prednisone), 12 (16%) had improved moderately, 12 (16%) had remained unchanged and two had deteriorated. If the early deaths are excluded the outcome is similar in the early and the late onset group without TH. Patients with TH had a less favourable course. Associated autoimmune diseases were diagnosed in 25% (n = 58). Because most of these patients were treated with anticholinesterases only, the evolution of their clinical state represents the natural course of MG.