Objective To demonstrate the cause of visual loss (VL) caused by pituitary adenomas (PAs),through retrospective study on the magnetic resonance imaging (MRI) radiological materials of patients with PAs.Methods Forty eight patients with PAs were selected,who received surgical treatment at our Hospital.Their medical records and MRI radiological films were reviewed.The relationship between the characteristics of PAs and the visual deterioration were analyzed.Neuro-ophthalmological findings were elevated with the relationship between the tumor and the supraelinoidal segment (C2 segment) of the internal carotid artery(ICA).Resuits On the coronal MR images,C2 segments of 28 sides(29.2%) were involved by the tumors whose value of VL in the ipsilateral eyes averaged 0.80±0.20 (range from 0.40 to 1.00).In the rest 68 sides (70.8%) without C2 segments involved,the valHe of VL in the correspondent eyes averaged 0.30±0.30 (range from 0.00 to 1.00).The difference was statistically significant (P<0.05).Conclusions If the C2 segment of ICA is invaded,the worse visual acuity would appear.which might be due to the markedly involved chiasmal arteries.