Freeze-fracture studies of caveolar density and distribution were carried out in skeletal muscle plasma membrane from 6 patients with Duchenne muscular dystrophy (DMD), 5 patients with facioscapulohumeral dystrophy (FSH), 5 patients with myotonic dystrophy (MyD) and 5 normal control subjects. The results showed a significant increase in the number of caveolae and a decrease in size in a population of muscle plasma membranes from patients with DMD. No significant changes in the caveolae were observed in muscle plasma membranes from patients with FSH and MyD.