A recombinant DNA study for deletion evaluation was performed in a 4 generation family with Duchenne muscular dystrophy (DMD) in twins. The patients were 6 years old, had a history of progressive difficulty in walking since age 4, and showed weak gluteals, iliopsoas, latissimus dorsi, rhomboids, lower trapezius, sternocleidomastoids, pseudohypertrophic calves, and tight heelcords. Both patients had high serum creatine kinase of 19,000 and 11,000 IU, respectively, and the muscle biopsy of the left vastus lateralis showed dystrophic alterations. Both twins had the same red cell types for ABO, Rh, CDE, MNSs, Kelly, Lewis, Duffy, and Kidd. HLA typing also detected the same antigens in both twins: A2, B44, DR4, and DR5. Cytogenetic studies were consistent with 46, XY male individuals with normal banding pattern. By cDNA probes the entire DMD gene was surveyed for missing or abnormal-sized restriction fragments. Both twin boys showed absence of 8.5, 8.0, 4.6, 4.2, and 3.1 kb fragments on Hind III blots and absence of 13.5, 3.7, 2.9, and 1.4 kb fragments on Bgl II blots both hybridized with cDNA 1-2a corresponding to most 5' region of the DMD gene. The mother and other relatives of the patient did not show deletion. These findings strongly suggest that the deletion in the DMD monozygotic twins represents a new mutation.