Objective: Pituitary adenomas are common neurological lesions believed to account for 10% to 15% of all primary brain tumors. There can be diagnostic confusion due to discordance of the preoperative endocrine and the postoperative immunohistochemical diagnosis. In this study, the rate of discordance between preoperative and postoperative findings and their clinical implications were investigated. Methods: From March 2005 to March 2006, 26 patients who underwent surgery for a pituitary adenoma were enrolled in this study. The preoperative pituitary hormone level and postoperative immunohistochemical results were compared and analyzed. Results: The median age of the patients was 38 years [range 15-66 years]. The male to female ratio was 8 to 18. The endocrine evaluation showed 16 hormonally-active and 10 hormonally-inactive adenomas. The immunohistochemical findings showed : 13 prolactin-positive, 1 GH-positive, 1 FSH-positive, 8 pleurihormone-positive and 3 stain-negative adenomas. The percentage of discordance observed between the preoperative endocrine and postoperative immunohistochemical diagnosis was 54%. Nine of 10 endocrine non-functioning adenomas showed : 3 PRL positive, 1 GH positive, 2 PRL+GH positive, 1 TSH+FSH positive, 1 FSH+ACTH+PRL positive and 1 FSH+LH+PRL positive adenomas by immunohistochemistry. Three endocrine PRL+GH secreting adenomas showed 2 PRL positive and 1 FSH+GH positive by immunohistochemistry. One endocrine PRL secreting and 1 GH secreting adenoma showed 1 PRL+ TSH positive and 1 GH+PRL positive by immunohistochemistry, respectively. The diagnosis of the other 12 pituitary adenomas showed concordance. Conclusion : The results of this study showed 54% discordance rate between the preoperative endocrine and postoperative immunohistochemical diagnosis for pituitary adenomas.