DNA studies were undertaken following 53 requests from pregnant women at risk for Duchenne and Becker muscular dystrophy, including 32 in whom there was only 1 affected individual in the family (sporadic cases). The DNA restriction fragment length polymorphisms were informative in 51 of the 53 cases. In 10 of 25 pregnancies with male fetuses the risk to the fetus was reduced to 5% or less. Referral of possible carriers before onset of pregnancy is strongly advisable on both medical and economic grounds. The banking of DNA from affected individuals for future use in the estimation of risks to their relatives should be encouraged.