CONTEXT: Endolymphatic sac tumors (ELSTs) are associated with von Hippel-Lindau disease and cause irreversible sensorineural hearing loss (SNHL) and vestibulopathy. The underlying mechanisms of audiovestibular morbidity remain unclear and optimal timing of treatment is not known. OBJECTIVE: To define the mechanisms underlying audiovestibular pathophysiology associated with ELSTs. DESIGN, SETTING, AND PATIENTS: Prospective and serial evaluation of patients with von Hippel-Lindau disease and ELSTs at the National Institutes of Health between May 1990 and December 2006. MAIN OUTCOME MEASURES: Clinical findings and audiologic data were correlated with serial magnetic resonance imaging and computed tomography imaging studies to determine mechanisms underlying audiovestibular dysfunction. RESULTS: Thirty-five patients with von Hippel-Lindau disease and ELSTs in 38 ears (3 bilateral ELSTs) were identified. Tumor invasion of the otic capsule was associated with larger tumors (P = .01) and occurred in 7 ears (18%) causing SNHL (100%). No evidence of otic capsule invasion was present in the remaining 31 ears (82%). SNHL developed in 27 of these 31 ears (87%) either suddenly (14 ears; 52%) or gradually (13 ears; 48%) and 4 ears had normal hearing. Intralabyrinthine hemorrhage was found in 11 of 14 ears with sudden SNHL (79%; P < .001) but occurred in none of the 17 ears with gradual SNHL or normal hearing. Tumor size was not related to SNHL (P = .23) or vestibulopathy (P = .83). CONCLUSIONS: ELST-associated SNHL and vestibulopathy may occur suddenly due to tumor-associated intralabyrinthine hemorrhage, or insidiously, consistent with endolymphatic hydrops. Both of these pathophysiologic mechanisms occur with small tumors that are not associated with otic capsule invasion.