Trigeminal neuralgia and hemifacial spasm were false localizing signs in three patients with contralateral space-occupying mass lesions in the posterior cranial fossa. According to radiological observations, the brainstem was remarkably displaced and distorted toward the side contralateral to the mass in all three cases. In the two cases with trigeminal neuralgia, the fifth cranial nerve was embedded in a thick arachnoid membrane and strongly compressed and angulated between the brainstem and the petrous bone, but there was no apparent vascular involvement. In the case with hemifacial spasm, only the contralateral tumor was removed. Postoperatively, all three patients experienced complete cessation of the symptoms without recurrence. Based on the operative findings, the authors postulate that angulation and distortion of the axis of the cranial nerve due to the contralateral mass, accompanied by thickening of the arachnoid membrane around the nerve, play an important role in false localizing signs, particularly in patients with trigeminal neuralgia.