Well-established surgical goals for pituitary macroadenomas include gross total resection for noninvasive tumors and debulking with optic chiasm decompression for invasive tumors. In this report, we examine the safety, reliability, and outcome of intraoperative magnetic resonance imaging (iMRI) used to assess the extent of resection, and thus the achievement of preoperative surgical goals, during transsphenoidal microneurosurgery. Our magnetic resonance operating room contains a Hitachi AIRIS II 0.3-T, vertical-field open magnet (Hitachi Medical Systems America, Inc., Twinsburg, OH). A motorized scanner tabletop moves the patient between the imaging and operative positions. For transsphenoidal surgery, the patient is positioned directly on the scanner tabletop so that the surgical field is located between 1.2 and 1.6 m from the magnet isocenter. At this location, the magnetic field strength is low (<20 G), thus permitting the use of many conventional surgical instruments. Thirty consecutive patients with pituitary macroadenomas underwent tumor resection in our magnetic resonance operating room by use of a standard transsphenoidal approach. After initial resection, the patient was advanced into the scanner for imaging. If residual tumor was demonstrated and deemed surgically accessible, the patient underwent immediate re-exploration. iMRI was performed successfully in all 30 patients. In one patient, iMRI was used to clarify the significance of hemorrhage from the sellar region and resulted in immediate conversion of the procedure to a craniotomy. In the remaining 29 patients, initial iMRI demonstrated that the endpoint for extent of resection had been achieved in only 10 patients (34%) after an initial resection attempt, whereas 19 patients (66%) still had unacceptable residual tumor. All 19 of these latter patients underwent re-exploration. Ultimately, re-exploration resulted in the achievement of the planned endpoint for extent of resection in all of the 29 completed transsphenoidal explorations. Operative time was extended in all cases by at least 20 minutes. iMRI can be used to safely, reliably, and objectively assess the extent of resection of pituitary macroadenomas during the transsphenoidal approach. The surgeon is frequently surprised by the extent of residual tumor after an initial resection attempt and finds the intraoperative images useful for guiding further resection.