The type and frequency of diagnostic testing was analyzed in a population-based cohort of boys with Duchenne muscular dystrophy or Becker muscular dystrophy. Use of muscle biopsy declined from 66.0% of boys born between January 1982 and September 1987 to 32.6% born between April 1999 and September 2004. DMD mutation was documented for 345 (73.4%) boys. Deletions were more common and point mutations were less common than that has been reported in specialty clinic or laboratory-based cohorts. Deletion of one or more exons was detected in 270 individuals (57.4% of all patients and 78.3% with a DMD mutation). Duplication was identified in 39 individuals (8.3% of all patients and 11.3% with a DMD mutation). Point mutation, small insertion, or small deletion was found in 36 individuals (7.7% of all patients and 10.4% with a DMD mutation). Point mutation analysis was performed in only 37 of 130 (28.5%) individuals with negative deletion and/or duplication testing.