Background: Cerebrospinal Fluid (CSF) leak during endonasal endoscopic pituitary adenoma surgery is an important factor that affects the way of sellar floor closure, postoperative CSF leak rate and postoperative outcome. Our objective is to determine the rate and significance of intraoperative cerebrospinal fluid leak in endoscopic pituitary Adenoma surgery with intrathecal fluorescein administration. Methods: The study focused on 18 patients diagnosed with pituitary adenoma. These patients underwent resection through endonasal transsphenoidal approach with the use of intrathecal fluorescein injection. The rate of intraoperative CSF leak was noted and correlated with tumor size, consistency, extension, extent of resection, postoperative CSF leak and the way of closure of the sellar floor. Results: With the use of intrathecal fluorescein, intraoperative CSF leak appeared to be higher than usual. Intraoperative CSF leak occurred in 15 patients (83.3%). As regard grades of CSF leak, 11 patients had grade 1(61.1%), 3 patients had grade 2(16.7%) and only one patient had grade 3 (5.6 %). Fluorescein injection was done for patients with no CSF leak or suspicion of CSF leak but not done for grade 2 and 3, hence, it was done for 14 patients (77.8%) and in 7 of them (50%), CSF leak appeared only with Fluorescein, in 4 of them (28.5%) suspicious leak was confirmed with Fluorescein and 3 of them (21.5%) had no CSF leak confirmed with no Fluorescein appearance after injection. Postoperative CSF leak occurred in 3 of 18 patients (16.7%) and relieved in all patients after few days with lumbar drain kept in place till CSF leak stopped. Conclusion: The use of intrathecal fluorescein during endoscopic transsphenoidal pituitary surgery is very helpful in identifying intraoperative CSF leak especially G1 leak, resulting in better sellar closure and less postoperative CSF leak. Keywords: Cerebrospinal Fluid Leak; Fluorescein; Intrathecal; pituitary adenoma