a five-years long study aiming to describe the basic genetic epidemiology of the dystrophinopathies in costa rica recruited 31 patients with clinical symptoms of dmd/bmd at the national children？s hospital (hnn). this center is the obligate reference hospital of the national health system for genetic diseases, however, the geographic origin of the patients, a low percentage of deletions and a high proportion of de novo mutations found among them indicate that a significant ascertainment bias impedes a substantial scientific approach to confront and alleviate the problems posed by these severe diseases in costa rica. rev. biol. trop. 52(3): 485- 490. epub 2004 dic 15.