The level of plasma brain natriuretic peptide (BNP) was elevated in 8 of 15 female gene carriers of Duchenne muscular dystrophy (DMD), and the level correlated with indices of cardiac function. In one of these carriers, whose clinical course was followed for one year, the plasma BNP level was elevated before the development of cardiac symptoms, further increased with the evolution of cardiac symptoms, and then decreased after treatment for cardiac failure. These results suggest that the plasma BNP level may be useful for the early detection of cardiac dysfunction and for evaluating the efficacy of cardiac treatment in female DMD carriers.