Objectives/Hypothesis: Sudden hearing loss (HL) can be caused by autoimmune disorders localized to the inner ear or secondary to systemic immune diseases. Studies in autoimmune animal strains showing HL have reported changes in the cochlear stria vascularis. The authors investigated the presence of antiendothelial cell antibodies (AECA) to see if immunemediated vasculitis may play a role in human sudden HL. Study Design: A prospective study in patients with sudden HL. Methods: Fifteen consecutive patients (mean age, 32 y) affected by sudden HL and 14 normal subjects were included. Patients with familial deafness and metabolic diseases were excluded. Extensive audiovestibular, imaging, microbiological, immunological, and routine examinations were performed. AECA were detected on rat kidney tissue sections on the sera collected at −20°C. Results: AECA were positive in 8 of 15 patients (53%) (2 of 5 men and 6 of 10 women), thus differing significantly from the normal control population, in which only 2 of 14 tested AECA positive (P = .023). Conclusions: In patients with sudden HL, immune-mediated vascular damage can have a pathogenetic role and AECA might represent a serological marker of vasculitis.