Eighty unrelated individuals with Duchenne muscular dystrophy (DMD) or Becker muscular dystrophy (BMD) were found to have deletions in the major deletion-rich region of the DMD locus. This region includes the last five exons detected by cDNA5b–7, all exons detected by cDNA8, and the first two exons detected by cDNA9. These 80 individuals account for approximately 75% of 109 deletions of the gene, detected among 181 patients analyzed with the entire dystrophin cDNA. Endpoints for many of these deletions were further characterized using two genomic probes, p20 (DXS269; Wapenaar et al.) and GMGX11 (DXS239; present paper). Clinical findings are presented for all 80 patients allowing a correlation of phenotypic severity with the genotype. Thirty-eight independent patients were old enough to be classified as DMD, BMD, or intermediate phenotype and had deletions of exons with sequenced intron/exon boundaries. Of these, eight BMD patients and one intermediate patient had gene deletions predicted to leave the reading frame intact, while 21 DMD patients, 7 intermediate patients, and 1 BMD patient had gene deletions predicted to disrupt the reading frame. Thus, with two exceptions, frameshift deletions of the gene resulted in more severe phenotype than did in-frame deletions. This is in agreement with recent findings by Baumbach et al. and Koenig et al. but is in contrast to findings, by Malhotra et al., at the 5'end of the gene.