Fourteen patients with Duchenne's muscular dystrophy (DMD) received nasal ventilation at the beginning of the declining phase of their vital capacity (VC). At that point, the mean value (SD) of their vital capacity was 1,756 (363) mL notably inferior to the predicted values. Nasal ventilation was performed according to a precise protocol, with an average nocturnal duration of 8 h. The annual decline of vital capacity was about 70 ml, showing a significant reduction compared with the value of 200 mL.yr-1 of untreated patients. Follow-up of treated patients (3.8 yrs; SD 1.9 yrs) has enabled us to distinguish different groups of results with useful practical consequences. In 29% of cases, the initial progression of decline of vital capacity was poorly modified by the treatment. In 71% of patients, the decline of VC was significantly decreased. In 35% of cases tracheal ventilation followed nasal ventilation. The normal mortality rate of DMD has greatly changed as a result of this method.