We describe the development and validation of an outcome measure for patients with myasthenia gravis (MG) and show the correlation of the items with conventional MG measurements. In stage I, item generation, a group of methodologists, clinical experts generated a list of 56 items. The list was based on (1) a previous study on an MG sample, (2) clinical experience and (3) items proposed by MG patients. In stage 2, reduction of items, the list was reduced on the basis of results from field testing (41 patients completed the 56-item questionnaire). In stage 3, reliability and validity were assessed. A 25-item MG questionnaire (MGQ) was generated. Results were related to conventional measures of MG severity. Furthermore, the MGQ appeared reliable, sensitive and reproducible. The questionnaire was validated as an outcome measure.