Steinert disease, also known as myotonic dystrophy type 1, is a muscle disease characterized by myotonia and by multiorgan damage that combines various degrees of muscle weakness, arrhythmia and/or cardiac conduction disorders, cataract, endocrine damage, sleep disorders and baldness. It is the most frequent of the adult-onset muscular dystrophies; its prevalence is estimated at 1/20,000 inhabitants. The disease is associated with abnormalities at the 19q13-2 locus (abnormally high CTG triplet repetition). Transmission is autosomal dominant, and anticipation may occur, that is, disease may be more severe and occur earlier in offspring. Detection of the 19q13-2 anomalies using molecular genetic techniques confirms the diagnosis. Genetic counseling is often delicate for this condition because of the wide variability in clinical expression, both within and between families. Prenatal diagnosis is proposed especially for maternal transmission because of the severity of the possible neonatal forms. Management ideally includes multidisciplinary annual follow-up. Disease course is usually slowly progressive but rapid deterioration may sometimes be observed. Life expectancy is reduced by the increased mortality associated with the pulmonary and cardiac complications.