In a case-control study the role of hyponatremia in the hearing loss of preterm infants was investigated. One hundred and sixty-four premature infants treated at the neonatal intensive care unit were screened with transient evoked otoacoustic emission (TEAOE). In 32 infants TEAOE results indicated the need for further investigations. Auditory brainstem response was performed and 22 of 32 cases had bilateral hearing impairment (HI). The birth weight and gestational age in the HI group were 1,425 +/- 528 g and 30.4 +/- 3.7 weeks. The matched control group consisted of 25 infants with a mean birth weight and gestational age of 1,410 +/- 280 g and 31.1 +/- 2.1 weeks. Significant differences were found between the HI and control groups: Apgar score (p < 0.05), pH value (p < 0.01) and pO(2) level (p < 0.05) were lower; the total dose of aminoglycosides (p < 0.01), furosemide usage (p < 0.01), the maximum pCO(2) level (p < 0.01), incubator stay (p < 0.05) and hyponatremia (p < 0.01) were higher, and the duration of hyponatremia (p < 0.05) was longer in the HI group. Multivariate logistic regression revealed that aminoglycoside treatment and hyponatremia were the two most significant factors in the development of hearing impairment. These results suggest that hyponatraemia is an additional risk factor for hearing loss in preterm infants.