By the use of a series of closely linked DNA probes detecting restriction fragment length polymorphisms (RFLPs) distributed over the short arm of the X chromosome, a double crossover was detected in a Duchenne muscular dystrophy carrier and an affected male fetus was diagnosed at 12 weeks of gestation, with a probable accuracy of more than 99.0%. A new mutation was identified in another family with the same degree of reliability; three females in this family were thus deemed not to be DMD carriers. The eleven RFLP-markers presently available on the short arm of the X chromosome are useful in the diagnosis of DMD since they bridge the Duchenne locus at genetic distances varying between 3 and 20 cmo. Moreover, recombination within the set of markers provides an independent way of regionally mapping these probes relative to each other along the short arm of the X chromosome.