About 60% of both Duchenne's muscular dystrophy (DMD) and Becker's muscular dystrophy (BMD) is due to deletions of dystrophin gene. For cases with deletion mutations the "reading frame" hypothesis predicts that deletions which result in disruption of the translation reading frame prevent production of stable protein and are associated with DMD. In contrast, intragenic deletions that involve exons encoding an integral number of triplet codons maintain proper reading frame. The resulting abnormal proteins are stable and partially functional, resulting in a milder and more variable BMD phenotype. To test the validity of this theory,we analyzed 40 patients-19 independent deletions at the DMD/BMD locus. Clinical/molecular correlations based on the altera-tions of the reading frame were valid in 69.2% of cases. After exclusion of: --2 patients with del 3-6 (with no consistent clinical expression); --1 DMD patient with large in-frame deletion; --2 patients that were too young to be classified; --4 patients in whom it was impossible to identify the extent of deletion (del 47 and del 44-45), the correlation between deletion and clinical severity was as predicted in 92.4% of cases. The present data should be useful in establishing the prognosis in individual patients even in sporadic cases with no affected relatives.