[Difficulties in genetic counseling in phakomatosis].

It is often difficult to establish a valid pedigree in a family, a member of which shows one or another phakomatosis. It is indispensable to examine all the members of the family in order to detect the "formes frustes" or the atypical and abortive forms, which are frequent, and in order to establish a genetic prognosis and to give a genetic counseling, taking the penetrance into account.