Forty-seven patients with thalassaemia have been studied to define risk factors for development of sensorineural hearing loss, and to establish guidelines for safe chelation. Sensorineural hearing loss was only present in patients who had previously received desferrioxamine (DFO). The two most significant risk factors were the maximum dose of DFO previously received (P less than 0.01), and a serum ferritin of less than 2000 micrograms/l at that time (P less than 0.001). A therapeutic index obtained from the ratio of the mean daily dose of DFO mg/kg divided by the serum ferritin identifies patients with a ratio of greater than 0.025 as at risk of sensorineural hearing loss (P less than 0.001) and can be used as a guideline for safe DFO dosage. Follow-up audiometry of the affected patients over a 2-year period indicated that adjustment of the dose to a therapeutic index of less than 0.025 resulted in the stabilization of hearing loss in seven patients and improvement in two.