In order to determine the effect of thymectomy on the prognosis of patients with myasthenia gravis, an analytical-descriptive study was carried out in the Neurological Clinic of the Groningen University Hospital and the Mathematical Institute of Groningen University. In 183 patients with generalized myasthenia gravis without thymoma, with onset between 9 and 46 years of age, the severity of the disease was scored by one neurologist at fixed times (0.5, 1, 2, 3, etcetera up to 15 years) after onset. Thymectomy was performed in 144 patients at different times (0.5, 1, 2, 3, etcetera up to 7 years) after onset. Logistic regression analysis was carried out with, as the criterion for marked improvement, a decrease of the disease score by 50%. Using this criterion, age, sex, duration of the disease and severity of the disease prior to operation played no distinct parts as prognostic factors. At different periods after the onset of the disease, groups of patients operated or not (yet) operated were compared. Patients subjected to thymectomy within 5 years after onset of myasthenia gravis had a better probability of halving of the severity score than patients not subjected to operation. These data render it possible in the individual patient to make an adequate estimate of the probability of marked improvement if thymectomy is performed or postponed.