Two cases of Moyamoya disease associated with persistent primitive trigeminal artery (PTA) are reported. Case 1: A 44 year-old man had sudden severe headache and found to have a subcortical hematoma in the left temporo-parietal lobe by CT-scans. Four vessel study revealed the right PTA and Moyamoya disease. Case 2: A 56 year-old woman suffered sudden severe headache and was diagnosed as subcortical hematoma in the right temporo-parietal lobe by CT scans. The left PTA and Moyamoya disease were revealed by four vessel study. In the previously reported 232 cases with PTA, we couldn't find out any case associated with Moyamoya disease. There are also no cases associated with Moyamoya disease in 93 cases of persistent primitive hypoglossal artery (PHA) whose vascular anomaly is essentially similar to that of PTA. In the 907 cases of Moyamoya disease collected by Japanese cooperative study, there were no cases in which PTA or PHA was associated by. The period when PTA or PHA disappears at the embryonic stage (5-14 mm) almost corresponds to the period (11-14 mm) in which the vascular state is similar to Moyamoya phenomena. From this point of view, two cases of Moyamoya disease associated with PTA suggest that there is a close relationship between Moyamoya disease and PTA. On the other hand, it seems that Moyamoya disease is associated with PTA by mere chance, because their combination is very rare and Moyamoya vessels changes frequently and dynamically, while PTA does not change.