OBJECTIVE To evaluate the preoperative MRI criteria of a sinus cavernous invasion by a pituitary adenoma. METHODS Study of 102 cavernous sinuses among 51 patients who had had a surgical cure of pituitary adenoma. Thirteen patients had a surgical invasion of the cavernous sinus. RESULTS A certain number of signs eliminated cavernous sinus invasion. The best means consisted in not crossing the intercarotid line (Sensitivity-Se = 100%, Specificity-Sp = 85% and Negative Predictive Value-NPV = 100%). The others means implied: not going past twelve o'clock on the internal carotid artery-ICA (NPV = 97.1%), symmetrical size of the cavernous sinus (NPV = 92.5%), non-convexity of the lateral wall (NPV = 90.2%), visualization of at least two venous groups of the laterosellar space (NPV = 90.2%) and finally, non-displacement of the ICA (NPV = 89.2%). The best criteria for diagnosis were passing by the intra and supracavernous ICA lateral tangent (Se = 84.6%, Sp = 95%) and the percentage of ICA encasement by the adenoma exceeding 25% (Se = 92.3%, Sp = 85%). CONCLUSIONS Except the total encasement of the intracavernous ICA, the cavernous sinus can be invaded when the lateral tangent of the supra and the intracavernous ICA is crossed, and also when the percentage of ICA encasement exceeds 25%.