Lafora's disease is included among the progressive myoclonic epilepsies. Despite the fact that dementia is a constant finding in this disease only a few papers have studied the timing of riental deterioration. We have performed wide neuropsychological testing in two cases early diagnosed as Lafora disease. The initial neuropsychological testing was carried out by the time there were no complaints of mental deterioration in both cases. In the first case consecutive neuropsychological testing demonstrated the rapidly progressive dementia. All neuropsychological testings in these cases showed severe impairment of right parietal lobe functions. Higher cortical functions related to language and intelectual processes were beet preserved in both cases. The functions related to constructional praxis, memory and abstract concepts and processes were severily impaired. Our data suggest that mental deterioration is an early manifestation in Lafora disease, even by the time normal social life is not yet disturbed. Dominant hemisphere cognitivo functions have been less impaired than the non-dominant ones. How a diffuse illness such as Lafora disease can cause such an asymmetrical higher cortical function deficit is not yet clear.