Hyperekplexia is commonly familial and with dominant transmission. The gene involved, GLRA1, encodes the alpha1 subunit of the glycine receptor. We describe 3 affected children homozygous for a new mutation, R100H. Both parents were heterozygous carriers; while the father was healthy, the mother has periodic limb movements during sleep. This suggests that Hys-100 could exhibit incomplete penetrance, but was linked to a severe classical form of hyperekplexia in homozygous.