Although the animal models used to characterize aminoglycoside ototoxicity are well developed, the initial stages of the ototoxic process of this important group of antibiotics in humans are less well understood. A group that receives frequent aminoglycoside therapy are cystic fibrosis (CF) patients, who may receive cumulative doses of gentamicin over 200 g in their lifetime. Consequently they represent a group in which it is particularly appropriate to monitor regularly auditory function. In this preliminary study, 15 young (aged 9-18 years) CF patients had their pure tone thresholds measured over 0.25-12 kHz. Their distortion product otoacoustic emission (DPOAE) growth functions were also measured at f2 = 2, 4 and 6 kHz with f2/f1 = 1.22. These results were compared with those obtained from 36 control volunteers of similar age. Fourteen of the CF patients had normal hearing (pure tone audiogram (PTA) thresholds < or = 10 dB HL over 0.25-8 kHz). In this group, there was a significant elevation of the stimulus levels required to generate a 2f1-f2 DPOAE < or = 10 dB SPL at 4 kHz. This elevation may represent one of the earliest changes in outer hair cell performance caused by gentamicin, although it may also be due to the CF condition itself. Whilst this measurement alone cannot be taken to indicate any serious cochlear dysfunction, it may have some clinical use as an early indicator or marker of functional deficit in the cochlea.