We report a 46-year-old, non-hypertensive man who suddenly developed isolated right trochlear nerve palsy. His diplopia was most prominent in the left lower gaze, and partially alleviated by head tilt to the left or by anteflexion of the neck. His CT scans showed a small high density area consistent with a hemorrhage in the lateral side of the right mesencephalic tectum. His MRI (T2-weighted images) showed a lesion consisting of mixed high- and iso-intensity areas with linear low intensity areas. The margin of the lesion was irregular and nodular. Cerebral angiography (prolonged injection) showed small feeding arteries (or capillaries) in the late arterial phase and dilated draining veins in the venous phase. No tumor stain, early draining veins, or capillary brushes were present. We thought he had an angioma (vascular malformation). AVM seemed unlikely. Review of the literature revealed that trochlear nerve palsy caused by a mesencephalic angioma is extremely rare. MRI and cerebral angiography (prolonged injection) seemed useful for the diagnosis of angiomas (Vascular malformations).