The internal flow dynamics were assessed in 16 cerebral aneurysms by cine phase-contrast magnetic resonance (MR) angiography. Six of the aneurysms were associated with subarachnoid hemorrhage and 10 were unruptured. The ruptured aneurysms demonstrated a 51% +/- 10% increase in volume between systole and diastole, while the increase in volume of the unruptured aneurysms was 17.6% +/- 8.9% (p < 0.005). The ruptured aneurysms demonstrated an approximate 1.0- to 1.5-mm increase in diameter in at least one dimension during systole. There was no relationship between aneurysm size and volumetric increases. These findings suggest that cine phase-contrast MR angiography can detect structural weakness in the aneurysmal wall. Imaging techniques that can evaluate aneurysmal wall dynamics may be useful in therapeutic planning.