RESUMEN
The Lafora disease is an uncommon genetic condition. Four cases (two families) were detected in Zarcero, a small town in Costa Rica (population under 2000). They belonged to two separate consanguineous marriages but both families had common ancestors. The diagnosis of Lafora disease was confirmed by liver and biopsy in one of the patients. The ages of onset were 13, 14, 16 and 17 years. Patients died after four, nine, six and five year of severe progressive physical and mental deterioration, respectively. The gene for Lafora disease arrive to Zarcero from one of its founders. There are no other cases reported from Costa Rica : this is an example of genetic drift, or more specifically, founder effect. Key words: Lafora disease, myoclonic epilepsy, founder effect