The natural history of mitochondrial diseases is poorly understood,limiting our ability to offer prognostic advice to patients or to evaluate therapy.One major obstacle to improving our understanding is the lack of a clinical rating scale to monitor the extensive clinical spectrum of mitochondrial disease.In this article,the authors describe the development and validation of a practical and semiquantitative rating scale,the Newcastle Mitochondrial Disease Adult Scale. The natural history of mitochondrial diseases is poorly understood, limiting our ability to offer prognostic advice to patients or to evaluate therapy. One major obstacle to improving our understanding is the lack of a clinical rating scale to monitor the extensive clinical spectrum of mitochondrial disease. this article, the authors describe the development and validation of a practical and semiquantitative rating scale, the Newcastle Mitochondrial Disease Adult Scale.