The aim of this study was to determine the effect of reducing mAs on the diagnostic quality of images and the radiation dose to the orbits in patients undergoing sinus CT. We studied 40 consecutive patients undergoing paranasal sinus CT for inflammatory disease prior to functional endoscopic sinus surgery (FESS). Four groups of 10 patients were scanned at 200 mAs, 150 mAs, 100 mAs and 50 mAs, respectively. Orbital radiation dose was measured using thermoluminescent dosemeters. Images were reviewed independently by two observers who were unaware of the mAs setting used. Image quality was evaluated using a semi-quantitative scoring system for six anatomical structures. The osteomeatal complex, uncinate process, infundibulum, frontal recess, middle turbinate and optic nerve were assessed as: clearly demonstrated (2 points); demonstrated but not clearly visualized (1 point); or not seen (0 points). No significant difference was shown between any of the four groups in terms of image quality according to the scoring system used in this study. Mean radiation dose to the orbit was reduced by 77%, from 13.5 mGy at 200 mAs to 3.1 mGy at 50 mAs (p<0.05). CT of the sinuses can be performed in patients prior to FESS at greatly reduced mAs without loss of diagnostic quality of the images. This is important in reducing the radiation dose to the lens.