OBJECTIVE: Posterior communicating arteries (PComAs) are innervated by vasodilatatory fibers of the trigeminal ganglion (TGG). We examined whether there is a relationship between the neuron density of the TGG and the severity of PComA vasospasm in a subarachnoid hemorrhage (SAH). METHODS: This study was conducted on 20 rabbits. Five were used as a baseline control group. Five were used as a sham group by injecting 1 mL of serum physiologic, and experimental SAH was applied to 10 animals by injecting homologous blood into the cisterna magna. After 10 days, PComAs and TGGs were examined histopathologically. PComA volumes and the neuron density of TGGs were estimated stereologically, and the results were analyzed statistically. RESULTS: In control group, the mean volume of the PComAs was 66,500±8500 μm, and the mean neuronal density of the TGGs was 8650±950/mm. In the serum physiologic group, the mean volume of the PComAs was 65,000±6550 μm, and the mean neuronal density of the TGGs was 8600±800/mm. In the SAH group, the mean volume of the PComAs was 46,500±5500 μm, and the mean neuronal density of the TGGs was 4200±500/mm. The results reveal an inverse relationship between the neuronal density in the TGG and the severity of the PComA vasospasm. CONCLUSIONS: The neuron density of the TGG may be an important factor in the regulation of PComA volume and in the continuation of cerebral blood flow. Low neuron density in the TGG may play an important role in the pathogenesis of PComA vasospasm in SAH. © 2012 by Lippincott Williams & Wilkins.