PURPOSE: A variation in the circle of Willis is not so common, but the most frequent type is hypoplasia/aplasia of the precommunicating anterior cerebral arteries (A1 segment). We aimed to examine the incidence and the clinical significance of A1 segment hypoplasia/aplasia in neuropediatric patients. METHODS: We retrospectively studied children with A1 segment hypoplasia/aplasia in brain magnetic resonance angiography (MRA) and compared the clinical and radiological aspects between children with A1 segment hypoplasia/aplasia alone and with other variations in the circle of Willis. RESULTS: Among 301 patients, 34 patients (11.3%) had A1 segment hypoplasia/aplasia. They presented neurological symptoms such as chronic headache, dizziness and visual disturbance. Seven (20.6%) had family history of neurological illness. Twenty seven (79.4%) had A1 segment hypoplasia/aplasia only, and seven (20.6%) had another vascular abnormality. Seven (20.6%) showed abnormal brain magnetic resonance angiography (MRI) results, cerebral atrophy being the most frequent (n=5, 14.7%). The incidence of abnormal brain MRI was 11.1% (n=3) in single vascular abnormality and 57.1% (n=4), significantly higher (p-value 0.02) in combined abnormality group. CONCLUSION: Structural alterations in the cerebral vasculature in children have important pathophysiological and clinical implications. Evaluation of variations in the circle of Willis, especially of A1 segment hypoplasia/aplasia using MRA is recommended.