An isolated case of Duchenne muscular dystrophy in a 15-year-old retarded girl with a de novo t(X;2) (p21.2;q37) translocation is described. Her clinical history is presented, with particular emphasis on CK level determinations made at several different times during her development. The similarities between this patient and previously reported females with Duchenne muscular dystrophy are discussed with reference to de novo translocations with a breakpoint at Xp21 and the risk of DMD in such females.