Quality of life and prognosis of patients with myotonic dystrophy type 1 (MD1) often depend on the degree of lung function impairment. This study was designed to assess the respective prevalence of ventilatory restriction, hypoxaemia and hypercapnia in MD1 patients and to determine whether postural changes in lung function could contribute to the early diagnosis of poor respiratory outcome. Fifty-eight patients (42.6 ± 12.9 years) with MD1 were prospectively evaluated from April 2008 to June 2010 to determine their supine and upright lung function and arterial blood gases. The prevalence of ventilatory restriction was 36% and increased with the severity of muscular disability (from 7.7% to 70.6%). The prevalence of hypoxaemia and hypercapnia was 37.9% and 25.9%, respectively. Multiple regression analysis showed that the supine fall in FEV1 was the only variable associated with ventilatory restriction, hypoxaemia and hypercapnia. Our data indicate that supine evaluation of lung function could be helpful to predict poor respiratory outcome, which is closely correlated with hypoxaemia and/or hypercapnia.