The cause of sensorineural hearing impairment is unknown in a high proportion of patients. Since ischaemia is a possible factor, the relation between hearing threshold and blood viscosity, plasma viscosity, and haematocrit was investigated in 49 patients with idiopathic hearing loss, taking into account age, sex, smoking, and socioeconomic group. Hearing thresholds were unrelated to haematocrit or low-shear blood viscosity. Hearing impairment at high frequencies was directly related to high-shear blood viscosity and inversely related to plasma viscosity. The derived measure of red-cell rigidity was significantly related at all frequencies to hearing thresholds. A second study of 92 subjects from a population sample confirmed the inverse relation with plasma viscosity and lack of relation with haematocrit. Increased pure-tone thresholds appear to be related to increased red-cell rigidity; this may be an important factor in sensorineural hearing impairment with implications for diagnosis and prevention.