Hemifacial spasm (HFS) is defined as an involuntary twitching of the hemifacial muscles supplied by the facial nerve. It is mostly attributed to a vascular compression with the facial nerve. Familial HFS has been described in less than 10% of cases of sporadic HFS and usually develops after the age of 40. Young-onset HFS is a rare condition, generally presenting prior to the age of 30 with an estimated prevalence of 6.5% of sporadic HFS. No familial component has been described in small series of young-onset HFS. Here we report the cases of a patient and his mother who both developed right HFS in their early twenties. Both patients had a neurovascular contact between their right facial nerve and a branch of their right posterior inferior cerebellar artery. Our report explores how young-onset familial HFS may be the consequence of hereditary vascular variations.