Relationship between angiographical manifestations and operative findings of hemifacial spasm was studied in 100 cases. Vertebral angiography was performed, and Towne, straight AP, and lateral projections were routinely studied. The anterior inferior cerebellar artery (AICA) directly compressed the facial nerve root exit zone in 54 instances, the posterior inferior cerebellar artery (PICA) in 38, and the vertebral artery (VA) in 11. Compressions by multiple vessels were observed in 3 cases. Anatomical variations of the AICA and the PICA were classified into 3 groups according to their origins and their distributions of blood supply: Type I, normal distribution of AICA and PICA; Type II, common trunk anomaly with dominant AICA (basilar artery origin); and Type III, common trunk anomaly with dominant PICA (vertebral artery origin). In our cases, 35% of them showed normal distribution, 34% dominant AICA, and 35% dominant PICA. Analyses of the angiograms revealed significantly increased numbers of common trunk anomalies when compared with normal angiograms studied by Takahashi. In 60 of the 65 cases with common trunk anomalies, facial nerves were compressed by the main trunk or the branches of the dominant artery. There were 35 cases which belonged to Type I anatomical classification. They were subdivided into 2 groups according to the size of the AICA and PICA: 1. AICA greater than PICA, and, 2. PICA greater than AICA. In the AICA greater than PICA subgroup, the AICA was the offending artery in all but one case. In the PICA greater than AICA subgroup, the PICA was responsible in 9 of 17 cases. In 31 cases, angiograms showed a redundant VA with lateral elongation into the cerebellopontine angle.(ABSTRACT TRUNCATED AT 250 WORDS)