The clinical course and prognosis of Duchenne muscular dystrophy (DMD) was compared in patients with deletions of the gene for dystrophin (cDMD) and those without such deletions. A total of 24 patients was followed for at least 2 yrs. At age 12 the rating of the activities of daily life (ADL) and disease stage were less favorable in those patients with deletions of the gene for cDMD. At age 14, no difference in ADL and disease stage was observed between the two groups. The percent vital capacity was lower in those patients with the cDMD deficit. When the prognosis was evaluated by multivariate analysis of the data obtained at age 12, the percent of patients predicted as dying before the age of 20 was 40% for those without the cDMD deficit but 76% for those who were cDMD defective. None of the cDMD defective patients lived longer than 20 yrs, whereas 5 of 14 patients without the cDND deficit survived longer than 20 yrs. Disorders such as cardiac and respiratory failure were also seen more frequently in the cDND defective patients. These results suggest that patients with Duchenne muscular dystrophy with defective cDMD have more severe disease than those without cDMD deficit.