Endoscopic surgery of the sinuses is a valuable procedure for evaluating allergic or infectious chronic recurrent sinusitis. It allows assessment of the ostium and the state of the mucosa. Its complications are exceptional (rhinorrhoea, meningitis, orbital trauma), but require a detailed preoperative examination in order to reveal variations in sinus anatomy. The role of preoperative CT is therefore to analyse the type and the extent of the disease and individual anatomical variants. It is difficult to predict the depth and angle of introduction of the endoscope on the basis of the CT scan, which is performed in the coronal plane with variable angle. Ideally, only lateral scans would allow guidance of the introduction of the endoscope (hence the value of MRI or 3-D reconstructions).