Internal elastic lamina (IEL) is the most vital structure of the cerebral arterial wall. Longstanding weakness of the IEL due to hemodynamic stress is compensated by adaptive intimal thickening. Formation of cerebral arterial trunk aneurysm is assumed to be the result of a break in the equilibrium between hemodynamic stress and the condition of the IEL and intima. Cerebral arterial trunk aneurysms unrelated to the branching zones have several lesional patterns of IEL and the state of intima. There is a strong relationship between the pathological features of aneurysms and their clinical courses. We attempt to delineate the concept of cerebral arterial trunk aneurysms including 'dissecting aneurysms' and 'fusiform aneurysms'.