The inheritance of seven restriction fragment length polymorphisms detected by DNA probes has been studied in families with Duchenne and Becker muscular dystrophies (DMD and BMD). The probes used have all been mapped to the short arm of the X-chromosome, four being distal and three proximal to the disease loci located within the Xp21 region. Linkage analysis of the DNA polymorphisms in relation to the two disorders showed similar genetic distances. Data obtained from DMD and BMD families have been combined to give more precise values for the different recombination fractions. Combined use of these polymorphic DNA markers will be of practical value in the genetic counselling of women at risk for Duchenne and Becker muscular dystrophy.