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.