A 4-month-old child with a very rare association of primary (congenital) left cerebral hypoplasia, hypoplasia of the left internal carotid artery, and giant anterior communicating artery aneurysm was seen. The aneurysm was detected and treated before rupture. We briefly reviewed the literature on agenesis (hypoplasia) of the internal carotid artery, intracranial aneurysms in childhood, and primary cerebral hypoplasia. It is proposed that cerebral arteriography, cranial computed tomography, or magnetic resonance imaging be performed in all children with primary cerebral hemihypoplasia to establish an etiologic diagnosis and, more importantly, to detect and treat possible associated intracranial aneurysms before they become symptomatic.