BACKGROUND: Dystrophia myotonica (DM) is the most frequent adult-onset muscular dystrophy. Type 1 is caused by the cytosine-thymine-guanine (CTG) repeat expansion in the DM protein kinase gene. Respiratory muscle weakness and altered central ventilatory control lead to hypercapnia and lung volume restriction. PURPOSE: This study aims to review the respiratory involvement in DM patients and study its relation with genetics. METHODS: Retrospective study of patients with DM referred for respiratory assessment was made. Noninvasive ventilation (NIV) was considered to daytime hypercapnia or symptoms of nocturnal hypoventilation. RESULTS: Forty-two consecutive patients (37.9 ± 13.6 years) were evaluated. Mean CTG length was 642.8 ± 439.2 repeats. In the first evaluation, mean forced vital capacity (FVC) was 74.4 ± 20.2 %, maximal expiratory pressure (MEP) 35 ± 16 %, maximal inspiratory pressure 52 ± 23 %, peak cough flow (PCF) 327.3 ± 97.7 L/min, arterial pressure of oxygen 79.7 ± 11.3 mmHg, arterial pressure of carbon dioxide 45.5 ± 6.2 mmHg, overnight minimal peripheral oxygen saturation (SpO(2)) 79.6 ± 11.6 %, and apnea-hypopnea index 13.9 ± 9.9. CTG length was found to be related with MEP (r = -0.67; p = 0.001) and SpO(2) (r = -0.37; p = 0.039). NIV was started in 25 patients. Ventilated patients had lower FVC (2.19 to 3.21 L; p < 0.001) and PCF (285.3 to 388.5 L/min; p = 0.003) and more CTG repeats (826.6 to 388.5 repeats; p = 0.02). NIV compliance was poor in seven patients (28 %) and related with hypercapnia (r = 0.87; p = 0.002) and inspiratory positive airway pressure setting (r = 0.65; p = 0.009). Ventilation improved symptoms and nocturnal hypoventilation. Comparing the first and last evaluations, only PCF was significantly lower (275.0 to 310.8 L/min; p = 0.019). CONCLUSIONS: Ventilatory insufficiency is very common in patients with DM and CTG length may be useful to predict it. Prolonged NIV improves symptoms, nocturnal hypoventilation and maintains daily blood gases. Routine evaluation of PCF should not be forgotten and assisted coughing training provided.