OBJECTIVE: To determine whether obesity is a potential risk factor of primary spontaneous cerebrospinal fluid rhinorrhea (CSFR). DESIGN: Retrospective study. SETTING: University hospital. PATIENTS AND METHODS: The clinical data of 79 patients diagnosed with CSFR who had been treated at our hospital between 1991 and 2001 were assessed. The data of 61 (77%) of 79 cases were complete and could be used for this study. Patients were segregated according to the cause of their CSFR: 21 (34%) due to head trauma, 14 (23%) due to previous surgery, 7 (11%) due to congenital malformation, and 2 (3%) due to tumor adjacent to the anterior cranial fossa. Of the 61 subjects, 17 (28%) had CSFR without any detectable reason. This group was therefore designated as primary spontaneous CSFR. The body mass indexes (BMIs) of all patients were compared and statistically evaluated. RESULTS: The mean BMI (calculated as weight in kilograms divided by the square of height in meters) of the 17 patients with primary spontaneous CSFR was 34.87, which was significantly higher (P<.001) than the mean BMI of the other 44 patients (28.53). The mean BMI of the group of patients with CSFR due to previous surgery or trauma was significantly lower than the BMI of the group with primary spontaneous CSFR (P<.003), whereas in relation to the group afflicted with tumors and malformations, only a tendency (P<.28) was found. CONCLUSION: Our data suggest that obese patients are at an increased risk to develop primary spontaneous CSFR.