To analyze possible etiological factors of spontaneous cerebrospinal fluid (CSF) rhinorrhea and to assess the outcomes of endonasal endoscopic repair. Retrospective study. Academic neurosurgical hospital. Patients Twenty-one consecutive patients who presented with spontaneous CSF leak and underwent endonasal endoscopic surgery from January 1999 through December 2001. Preoperative examination included computed tomographic scans; nasal endoscopy; measurement of glucose concentration in the nasal discharge; and, in some cases, cisternographic evaluations via computed tomography and/or magnetic resonance imaging. Telescopes, conventional endoscopic sinus surgery instruments, and a microdebrider were used for all patients who underwent endonasal surgery. A combination of plastic materials, ie, abdominal fat, fascia lata, rotated middle turbinate flaps, and fibrin glue, were used for fistula repair. At the time of surgery, CSF fistulas were found in the cribriform plate (6 patients), in the fovea ethmoidalis (6 patients), and in the sphenoid sinus (9 patients). In 5 of the 6 patients who had an extremely pneumatized sphenoid sinus, the source of the leak was located in the lateral extension of the sinus. A meningocele protruding through the bone defect was the source of the leak in 10 patients. Postoperative follow-up lasted from 9 to 42 months, and 20 patients were considered cured. There was only 1 recurrence, in a patient whose CSF rhinorrhea originated in the deep lateral recess of an overpneumatized sphenoid sinus. Thus, the overall success rate was 95.2%. There were no postoperative complications. Possible etiological factors of this disease include obesity, congenital malformations of the skull base, an overpneumatized sphenoid sinus (particularly in its lateral extensions), and the empty sella syndrome. Endoscopic endonasal repair of spontaneous CSF rhinorrhea appears to be a safe and successful procedure. However, techniques for endoscopic closure of CSF fistulas in the lateral part of the sphenoid sinus need further perfecting.