Ten patients are reported with bilateral cochlear deafness which was associated with vitamin D deficiency. The features of these cases are discussed following an overview of the clinical aspects and diagnosis of vitamin D deficiency. The most likely pathogenesis is localized demineralization of the cochlea resulting in secondary morphological changes. Replacement therapy resulted in unilateral hearing improvement in two of the four patients in whom the response to treatment could be assessed. This suggests a previously unrecognized causal correlation between vitamin D deficiency and cochlear deafness. Impaired vitamin D activity may be important in the aetiology of otosclerosis, presbyacusis and the deafness associated with chronic renal failure. Vitamin D deficiency should be considered in the differential diagnosis of unexplained bilateral cochlear deafness. It is important, as this 'new' metabolic type of sensorineural deafness may be reversible, and may also lead to the diagnosis of early osteomalacia before more serious generalized skeletal symptoms can occur.