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We present 25 pituitary adenomas that were confirmed surgically to have invaded the cavernous sinus space. The surgical results are compared with the preoperative magnetic resonance imaging findings. For comparable radiological criteria, we classified parasellar growth into five grades. This proposed classification is based on coronal sections of unenhanced and gadolinium diethylene-triamine-pentaacetic acid enhanced magnetic resonance imaging scans, with the readily detectable internal carotid artery serving as the radiological landmark. The anatomical, radiological, and surgical conditions of each grade are considered. Grades 0, 1, 2, and 3 are distinguished from each other by a medial tangent, the intercarotid line--through the cross-sectional centers--and a lateral tangent on the intra- and supracavernous internal carotid arteries. Grade 0 represents the normal condition, and Grade 4 corresponds to the total encasement of the intracavernous carotid artery. According to this classification, surgically proven invasion of the cavernous sinus space was present in all Grade 4 and Grade 3 cases and in all but one of the Grade 2 cases; no invasion was present in Grade 0 and Grade 1 cases. Therefore, the critical area where invasion of the cavernous sinus space becomes very likely and can be proven surgically is located between the intercarotid line and the lateral tangent, which is represented by our Grade 2. We also measured tumor growth rates, using the monoclonal antibody KI-67, which shows a statistically higher proliferation rate (P < 0.001) in adenomas with surgically observed invasion into the cavernous sinus space, as compared with noninvasive adenomas.