Focal dysplasia of the right atrium was identified postmortem in a 22-year-old man with myotonic dystrophy and sudden death. Antemortem cardiac abnormalities included a single syncopal episode associated with atrial flutter with exercise-induced 1:1 atrioventricular conduction, sinus node dysfunction, and mild mitral valve prolapse. Pathologically there was only mild conduction system disease and the ventricular myocardium was normal. Right atrial dysplasia, previously unreported in myotonic dystrophy, appears to have been an arrhythmogenic lesion in this patient, serving as a morphological substrate for reentry.