Respiratory failure is the commonest cause of mortality in myotonic dystrophy type 1 (DM1). The variation in the DM1 phenotype causes difficulty in clinically predicting the severity of respiratory involvement. Parameters, such as daytime somnolence, are insensitive in highlighting DM1 patients who require non-invasive ventilation and are at risk of life threatening respiratory compromise. We have retrospectively analysed a cohort of 118 DM1 patients at the point of their first respiratory assessment and provide a multivariate model for predicting ventilator requirement based on the patient’s triplet repeat years score (derived from a semi-quantitative analysis of triplet repeat size and age of patient) and Muscle Impairment Rating Scale (MIRS). We also describe the causes of failure to tolerate NIV in DM1.