OBJECTIVE To identify potentially preventable risk factors for sensorineural hearing loss (SNHL) in extremely premature infants. METHODS A case control study of survivors with gestational age (GA) < 28 weeks or birthweight (BW) < 1000 g using data collected prospectively in our Neonatal Intensive Care Unit database. Each subject with bilateral SNHL > 40 dB was matched according to GA, BW and sex with two controls who had neither sensorineural nor conductive hearing loss. RESULTS Infants with SNHL had increased mean (+/- s.d.) days ventilated (53 +/- 21 vs 37 +/- 23 days, P = 0.006) and in oxygen (107 +/- 44 vs 69 +/- 28 days, P = 0.02) compared with controls. The risk for SNHL was increased for infants who spent > 90 days in oxygen (OR 4.0 [95% CI 1.1-15.6]), had maximum FiO2 > 0.90 (5.6 [1.2-26.9]), minimum plasma Na < 125 mmol/L (5.6 [1.1-27.8] or maximum pH > 7.60 (5.6 [1.1-89.0]). Neither maximum serum bilirubin nor exposure to ototoxic drugs was associated with SNHL. CONCLUSIONS Avoidance of severe hyponatraemia and extreme alkalosis, as well as use of surfactant to minimize the severity of hyaline membrane disease, may result in a decreased incidence of SNHL in extremely premature infants.