Presymptomatic and prenatal diagnosis of myotonic muscular dystrophy with linked DNA probes. Advances in neurogenetics are facilitating clinical care. Localization of the mutant gene that causes myotonic muscular dystrophy (DM) to chromosome 19 enabled our predictive testing using linked DNA probes in 74 members of 12 families at risk. Individuals sought either diagnostic confirmation or exclusion with childbearing in mind, or requested prenatal diagnosis. Valuable information was provided for 11 of 12 families. Of 14 individuals at 50% risk, 12 learned they did not have DM, two learned they did (although presymptomatic), and prenatal diagnoses of affected fetuses were made in three families--all with high degrees of certainty. The future opportunity for prenatal diagnosis was provided for 3 other families. The potential health risks to an affected female and her affected or nonaffected fetus provide cogent reasons for physicians to inform DM families in their care about these important advances and opportunities to avert grave complications. 