In extended and multigenerational pedigrees, the idiopathic epilepsy phenotype shows an extreme variability.The range of idiopathic epilepsy onset age in multigenerational pedigrees was studied in order to determine if genetic anticipation play a role in the heredity of Idiopathic Epilepsies.We compare the seizures onset age among relative-pairs of (parents-children, grandfathers-grandsons and nephew uncles). The mean onset age was compared using the Wilcoxon sign-rank paired-sample non-parametrical test to determine whether or not significant differences over > 0 exist, which refutes the null hypothesis of not anticipation. 84 pairs of relatives were taken from 72 extended multigenerational pedigrees.The onset age of idiopathic epilepsy of the pairs showed a difference significantly > 0, which confirm the existence of intergenerational differences. This difference has a tendency to decrease in age which each successive generation. This difference occur in all relative pairs and therefore contradicts the ascertainment bias described by Penrose.The results outline the existence of unstable mutations (those produced by a nucleotidic variable number of tandem repeats) as a probable explanation of the susceptibility to develop some forms of idiopathic epilepsy.