Myotonic dystrophy type 1 (DM1) is characterized by an unstable expansion of a CTG repeat resulting in altered mRNA biogenesis. Benign or malignant tumours are increasingly reported. The aim of the study was to evaluate the risk of tumor in a cohort of patients DM1. We retrospectively reviewed the medical records of every DM1 patient admitted in our neuromuscular center. Diagnoses of cancer and age at diagnosis were noted. The relative risk of a selected cancer was calculated using the data of the cancer registry obtained from the French "Institut de Veille Sanitaire". A total of 109 French DM1 patients, aged 44.1±13.0 years, were studied, and 14 malignant tumours were observed, with a significant relative risk (RR) of thymoma, of gynaecologic cancers, of lung cancers. While this cohort is small, our findings nevertheless suggest an increased risk of particular cancers in DM1. The toxic effects of mutant RNA may possibly affect oncogene expression or growth factor signalling pathways in cells. Clinical practice should include cancer screening and prevention of risk factors in DM1 patients.