Prenatal prediction of myotonic dystrophy (Dm) is feasible because Dm is closely linked to the secretor (Se) locus and the Se status of the. fetus can be determined by examination of the amniotic fluid. A pregiiant woman with Dm and her husband presented a favorable mating for prenatal diagnosis. A Se‐negative fetus would have been at high risk for Dm (92%, allowing for recombination). The fetus was found to he Se‐positive and pregnancy was not terminated. Overall, 37.5% of matings are potentially favorable for prenatal prediction by linkage. The affected parent must be heterozygous at the secretor locus; the spouse must he either se/se or potentially Se/se. Otherwise, prenatal diagnosis is impossible. Guidelines have been prepared for intrauterine prediction of myotonic dystrophy in matings of various Se genoty