The relations between angiographic manifestations and operative findings of hemifacial spasm were studied in 70 cases between 1988 and 2001. Vertebral angiography was performed, and Towne, straight AP, and lateral projections were routinely examined. The dominant anterior inferior cerebellar artery (AICA) directly compressed the facial nerve root exit zone in 26 cases, the dominant posterior inferior cerebellar artery (PICA) in 20, the AICA in 13, the PICA in 2, and the vertebral artery (VA) in 9. Compression by multiple vessels was observed in 11 cases. Anatomical variations of the affected AICA and PICA were classified into 3 groups according to their origins and distributions of blood supply: normal distribution of AICA and PICA in 18%, common trunk anomaly with dominant AICA (basilar artery origin) in 48% and common trunk anomaly with dominant PICA (vertebral artery origin) in 34%. Analyses of the angiograms revealed significantly increased numbers of common trunk anomalies compared with cases with normal angiograms. In 18 of the 20 cases of unilateral common trunk anomalies, facial nerves were compressed by the dominant artery. Preoperative vertebral angiograms may clarify the offending vessels and their sites in most hemifacial spasm cases, thus increasing the safety of surgical interventions.