<h4>Background</h4>Myotonic Dystrophy type 1 (DM1) is an autosomal dominant disease with anticipation due to increased number of CTG repeats in the DMPK gene.<h4>Methods</h4>This retrospective, cohort study in Iceland assessed prevalence of DM1, molecular pathology, and patient ascertainment. The gene discussed is DMPK; the disease is autosomal dominant disease.