One of the most important prognostic factors in brain tumours is the extent of tumour resection. Its evaluation has been difficult on computed tomography (CT); however, magnetic resonance imaging (MRI) can clearly determine the extent of tumour resection. Using MRI, we analyzed 77 patients with astrocytic tumours that were surgically treated at our department from 1994 to 2001. The tumours were classified into the following types: (a) well-circumscribed: single lesions that can be distinguished from normal brain; (b) localised: single lesions that are localised in one gyrus; and (c) diffuse: other tumours. Our treatment of glioma resection is to use sulcus opening and gyrectomy, a technique based on identification and dissection of the sulcus adjacent to the tumour followed by en bloc tumour resection. Almost total tumour resection (> or =95% resection) was achieved in 76.8% in the well-circumscribed type, 100% in the localised type, and only 10.5% in the diffuse type. Nearly total resection was achieved in 61%, which is considerably more than in the literature of the CT era (10.4-23.5%). The sulcus opening and gyrectomy technique based on MRI achieved radical glioma removal in cases which could be identified by MRI, resulting in a better tumour removal rate than that based on CT.