Intrathecal injection of fluorescein is a useful adjuvant method for localization of fistulas in endoscopic endonasal repair of cerebrospinal fluid (CSF) leakage. Although being neurotoxic in commercial doses, a low dose of diluted fluorescein seems to be safe on the basis the existing data in the literature. The purpose of this study was to investigate the role of a low- dose intrathecal fluorescein injection in detection of CSF fistula and potential adverse effects of this technique. CSF rhinorrhea was repaired in 20 patients with an endoscopic endonasal technique. Intraoperative intrathecal fluorescein injection was used for localization of the site of the CSF leak. The accuracy rate of leakage site identification and the incidence of complications and recurrences were recorded. Intrathecal administration of fluorescein demonstrated CSF leakage in 18 of the 20 patients (90%). There were no intraoperative complications. Definitive closure of the CSF leakage site was achieved in 16 patients (80%) after the initial reconstruction. Recurrence occurred in four cases and all patients were free of CSF leakage after the second surgical attempt. The present study suggested that a low dose of intraoperative intrathecal fluorescein administration is a safe and sensitive method for localization of CSF leakage sites.