The effect of 12 weeks of aerobic training on a cycle ergometer was studied in 12 patients with myotonic dystrophy. Efficacy was evaluated by cycle testing and muscle morphology before and after training. Patients increased their maximal oxygen uptake by 14%, the maximal workload by 11%, muscle fiber diameter increased significantly, and creatine kinase did not increase with training. The study indicates that aerobic training is safe and can improve fitness effectively in patients with myotonic dystrophy.