AIM: To evaluate whether narrowing of internal carotid artery siphon (ICAS) may increase the risk of developing non-arteritic anterior ischaemic optic neuropathy (NAION). METHODS: Totally 30 consecutive patients who had unilateral NAION and 30 gender-matched control subjects were recruited in the present study. The diameter of ICAS of all the participants were measured using head-and-neck computed tomographic angiography (CTA). Color doppler flow imaging (CDI) was used to measure the haemodynamics parameters of ICAS and short posterior ciliary arteries (SPCAs) in all subjects. Comparison of parameters between the NAION patients and controls as well as between the two sides within the patients were performed. The correlation between the diameter of ICAS and NAION was analyzed. RESULTS: A comparison of parameters between the affected side of the NAION patients and the controls, including the diameter of ICAS, the resistance index (RI) of ICAS, the blood flow velocities of SPCAs and RI of SPCAs, showed significantly difference (P<0.01), while there was no significant difference in terms of the mean blood flow velocity (Vm) of ICAS; Similar results were found while comparing all the measurements of the affected and unaffected side of patients (P for RI of SPCAs <0.05). No marked difference was detected in nearly all parameters except for RI of ICAS and SPCAs between the unaffected side of the NAION patients and the controls (P<0.05). The diameter of ICAS were significantly positive correlated with both peak systolic velocity (PSV) of SPCAs and end diastolic velocity (EDV) of SPCAs in patients with NAION (r=0.514, P<0.01 and r=0.418, P<0.05, respectively). CONCLUSION: Narrowing of ICAS may increase the risk of developing NAION.