Becker muscular dystrophy (BMD) is characterized by progressive muscle weakness, with a large variation of disease progression. To investigate possible causes for this diversity it is necessary to compare the disease course between different BMD patients. Due to the rarity of the disease, it is difficult to compose and study large groups of patients of the same age. Therefore we have developed a model to enable comparison of disease severity in BMD patients of different ages. For this purpose, Dutch and Flemish DNA-confirmed BMD patients were interviewed using a standardized questionnaire focusing on the age at reaching five disease milestones: first motor symptoms, difficulty walking stairs, use of a walking aid, wheelchair dependence and mechanical home ventilation. Each patient received 10 points for the age at which a specific milestone was reached and an individual composite score was calculated for each age. Combining the results of all participants, a population mean score was calculated at each age individually. Information was gathered from 113 BMD patients, ranging in age from 6 to 66 (median 39) years. Estimated mean survival for reaching the individual disease milestones was: first motor symptoms 11years (SE 0.92; n=109), difficulties walking stairs 25years (SE 1.5; n=90), use of a walking aid 37years (SE 1.8; n=71), wheelchair dependence 50years (SE 2.0; n=35) and mechanical home ventilation 63years (SE 1.2; n=5). Out of all individual data a model for disease severity was calculated. The maximum score in this model was 50 points. Mean population scores at age 10, 20, 30, 40, 50 and 60years respectively were 8.4, 16.7, 20.2, 23.7, 26.8 and 30. When combining these mean scores with their SD, individual Z-scores can be calculated, enabling comparison of disease severity in BMD patients of different ages. As long as no prospectively collected long-term follow-up data are available, this scale could form a useful tool for clinical research in BMD.