Thirty-seven of 317 patients with pituitary adenoma who underwent transsphenoidal operation from 1989 to 1996 received adjuvant gamma knife radiosurgery. Gamma knife surgery was performed mainly in patients with endocrinologically inactive tumor for tumor regrowth invading the cavernous sinus, and in patients with endocrinologically active tumor for incomplete removal invading the cavernous sinus. The maximum radiation dose was 25-60 Gy. The periphery of the tumor usually received 50% of the maximum dose. Thirteen patients were followed up for longer than 2 years (mean 45 months) after combined therapy. Magnetic resonance imaging (MRI) showed changes in signal intensity on both T1- and T2-weighted images as early as 3 months after radiotherapy. Serial MRI showed all 13 patients had achieved excellent response. Patients with endocrinologically active tumors showed normalized hormone levels 24 months after gamma knife surgery except for one patient with acromegaly. The basal value of pituitary hormones remained normal during the follow-up period, and four female patients became pregnant without hormonal therapy. Combined transsphenoidal surgery and gamma knife radiosurgery can preserve normal pituitary function and eradicate adenoma invading the cavernous sinus.