The genetic basis of muscular dystrophy in golden retriever dogs was investigated by means of experimental matings and cytogenetic studies. An affected male golden retriever was mated to three normal females, producing an F1 generation of six males and 14 females, all of which were clinically normal. Of six F1 females retained for breeding, all were shown to be carriers of muscular dystrophy in outcrosses to unrelated normal male dogs or in backcrosses to the affected male golden retriever. In outcrosses of carrier females, three of seven male and none of nine female offspring were affected, as expected under the X-linked recessive hypothesis. Backcrosses of F1 females to their affected sire also yielded results that are consistent with this hypothesis: 15 of 32 males and 5 of 17 females had muscular dystrophy. Cytogenetic studies of a carrier female, an affected male offspring, and a normal male sibling revealed no detectable abnormalities of the X chromosome.