The electroretinograms (ERGs) of patients with definite myotonic dystrophy were studied as well as neurologically asymptomatic patients with minimal expression of myotonic dystrophy. Children in 4 families exhibited definite myotonic dystrophy when neither parent exhibited clinical or electromyographic signs of myotonic dystrophy. Myotonic dystrophy is dominantly inherited. We were able to identify the asymptomatic parent with the gene in these families. This study suggests that ERG testing, which includes scotopically balanced dim blue and red flashes, will identify some asymptomatic adults minimally expressing the gene for myotonic dystrophy.