Purpose : To describe clinical and MRI findings of growth hormone-secreting pituitary adenoma, to determine ifthere are any characteristic MRI findings different from those of other pituitary adenomas, to evaluate therelationship between tumor size and serum growth hormone level, and to assess the results of immunohistochemicalstudy. Materials and Methods : We retrospectively analysed clinical and MRI findings of 29 patients with growthhormone-secreting pituitary adenoma confirmed by serum growth hormone level and surgery. We also evaluated therelationship between the tumor volume and serum growth hormone level, and the results of immunohistochemicalstudy. Coronal and sagittal T1-weighted MR images in all patients and gadoliniumenhanced T1-weighted MR images in28 patients were obtained with 2.0 T(24 cases) and 0.5 T(5 cases) MR imagers. The images were analyzed in terms oftumor size, signal intensity, degree of contrast enhancement, extent of tumor growth and the presence or absenceof cystic change, hemorrhage and calcification. Results : Clinical manifestations included facial feature changeand soft tissue swelling of hands and feet(n=29), headache(n=12), impaired visual acuity(n=9), symptoms ofhyperprolactinemia(n=8), visual field defect(n=5), and others(n=6). On MR images, all of the 29 cases were seen tobe macroadenomas and the size of the tumors averaged 2.2cm(1-5.2 cm). Supra- and infrasellar extensions were seenin 21 and 22 patients, respectively. Cavernous sinus invasion was noted in seven, and in one this was bilateral.Signal intensity was isointense with cortical grey matter in 26 cases(90%). Cystic change or necrosis was seen ineight cases(28%), hemorrhage in four(14%), and calcification in two(7%). After enhancement, most(25/28) of thetumors enhaned less than mormal pituitary in degree. There was no correlation between serum growth hormone leveland tumor size. Immunohistochemical study showed positive growth hormone-secreting cells in only 69%(11/16).Conclusion : Clinical findings in patients with growth hormone-secreting pituitary adenomas were various andincluded acromegaly, headache, visual impairment, and symptoms of hyperprolactinemia, for example. On T1-weightedMR images, all tumor were macroadenoma, and there were no characteristic findings different from those of otherpituitary tumors. The volume of a tumor did not correlate with serum hormone level. Immunohistochemical studyshowed positive growth hormone-secreting cells in a limited number of cases.