BACKGROUND: Previous studies indicate that vital capacity in patients with Duchenne muscular dystrophy increases with age when they are under 12 years old, and decreases from 13 or 14 years of age;however, recent studies indicate that the vital capacity in patients with Duchenne muscular dystrophy begins to decrease even before 12 years of age.OBJECTIVE: To verify if the vital capacity in patients with Duchenne muscular dystrophy decreases before the age of 12 years and to observe the effect of rehabilitation exercise on vital capacity.DESIGN, TIME AND SETTING: The case analysis was performed at the Department of Neurology,The First Affiliated Hospital of Sun Yat-Sen University (Guangzhou, Guangdong Province, China) from December 2004 to January 2006.PARTICIPANTS: Sixty-five male patients diagnosed as having Duchenne muscular dystrophy and who underwent pulmonary ventilation function examination at the Department of Neurology, The First Affiliated Hospital of Sun Yat-Sen University (Guangzhou, Guangdong Province, China) from December 2004 to January 2006; ages ranged from 6 to 22 years old.METHODS: The ventilation function of 65 patients was determined using a Sensor Medics 2100pulmonary function test apparatus (USA), and the data obtained were subjected to statistical analysis comparing patients under 12 years of age and those above 13 years of age, and comparing those who performed rehabilitation exercise with those who did not.MAIN OUTCOME MEASURES: Forced vital capacity (FVC); forced expiratory volume in one second (FEV1); maximal voluntary ventilation (MMV); the ratio of forced expiratory volume in one second and forced vital capacity (FEV1/FVC); each measured value as a percentage of the corresponding predicted value.RESULTS: There were no significant differences in FVC, FEV1 and MMV between patients under 12 years of age and those above 13 years of age (P > 0.05). The FVC, FEV1 and MMV values, as percentages of the predicted values, were, in patients under 12 years old, significantly higher than those in patients older than 13 (P < 0.05). All ventilation function parameters except FEV1/FVC in patients undergoing rehabilitation exercise were higher than those in patients without rehabilitation exercise. This difference was not significant in patients under 12 years of age, but was statistically significant in those older than 13 years (P < 0.05).CONCLUSION: Although the values of ventilation function measured increase with age in Duchenne muscular dystrophy patients less than 12 years of age, real ventilation function is already damaged.Thirteen years of age is an important time point for pulmonary function change. Rehabilitation exercise can slow down the process of pulmonary function exacerbation in Duchenne muscular dystrophy patients,especially when therapy starts before 12 years of age.