Pituitary adenomas are usually soft, but 5-13.5% are fibrous adenomas which are difficult to remove. Magnetic resonance (MR) imaging and operative findings were evaluated in eight patients (12.1%) with fibrous pituitary adenoma among 66 patients. Tumor specimens were examined histologically and immunohistochemically for collagen content and subtypes. Seven patients had clinically inactive non-functioning pituitary adenomas and one patient growth hormone-secreting adenoma. All patients underwent transsphenoidal surgery. Four cases were giant adenomas with suprasellar extension of more than 2 cm. T1- and T2-weighted MR imaging showed the tumors as nearly isointense to the surrounding brain, except in one case where the tumor was high intense on T2-weighted MR imaging. All tumors required piecemeal resection using a micro-dissector and tumor forceps. Four tumors of maximum size more than 3 cm needed a second operation. The interface between the thinned normal pituitary gland and fibrous adenoma was intended to identify at the anterior-superior portion in recent four cases, which was helpful to remove the tumors and preserve pituitary functions. Histological examination revealed prominent deposition of collagen in the perivascular area. The percentage of collagen content in fibrous adenomas was more than 5% and significantly higher than that in soft adenomas and normal pituitary glands. Immunohistochemical examination showed positive staining for collagen types I and III in the fibrous adenomas, but only for type V collagen in the normal pituitary glands. Large fibrous adenomas can be resected by transsphenoidal surgery which may require two-stage operations. Identification of the interface between the normal pituitary gland and adenoma is helpful to remove fibrous adenomas and to preserve pituitary functions. We propose that firm adenomas containing more than 5% collagen are "fibrous" adenomas.