Deafness appears in as many as 4% of adult cases of epidemic parotitis. It is often severe, though reversible and is usually unilateral. Vertigo has been reported in mumps, as have occasional cases with impaired caloric reaction. Twenty cases of unilateral hearing impairment in mumps have been investigaed with audiometry and electronystagmography in order to determine the degree of permanent lesions. Nine of the patients had noted vertigo when falling ill with protitis. Five of these had normal calorics, 3 were impaired and one was without response in the deaf ear. No certain correlation between vertigo and permanent caloric impairment was found. An interesting finding was that 5 cases without vertigo showed an impaired or absent caloric responsiveness, which might confuse future diagnostics. Presumably, most patients with hearing impairment in mumps suffer vesticular damage, but the acute vertigo in early childhood is easily overlooked. No certain correlation between age at the onset of mumps and any permanent caloric disturbance was found. One mumps case is described, in which a severe hearing loss and caloric impairment returned to normal.