We hypothesized that changes in brain-stem auditory evoked responses related to bilirubin would be associated with changes in cry because of the anatomic proximity in the brain stem of cranial nerves 8 (auditory) and 9 to 12 (vagal complex, which controls cry). Brain-stem auditory evoked responses and computerized cry analysis were used to study the concurrent effects of moderate hyperbilirubinemia on auditory function and cry. Fifty term infants were divided equally into two groups on the basis of serum bilirubin concentrations: low (less than 8 mg/dl; 136) mumol/L and moderate (10 to 20 mg/dl, 170 to 342 mumol/L). Forty-three infants had successful tracings of brain-stem auditory evoked responses recorded with a Cadwell model 5200A evoked response unit during two successive trials, and a cry recording of each infant was analyzed by computer. The moderate serum bilirubin group had an increase in percent cry phonation (p less than 0.02) and an increase in the variability of the first formant (p less than 0.04) in comparison with the low serum bilirubin group. Serum bilirubin values correlated positively with brain-stem conduction time (r = 0.36, p less than 0.01), percent phonation (r = 0.42, p less than 0.004), and variability of the first formant (r = 0.39, p less than 0.02). Percent phonation, the voiced component produced by increased neural control, correlated with the interpeak of waves latencies I to III (r = 0.32, p less than 0.03) and brain-stem conduction time (wave I to V) (r = 0.35, p less than 0.01). We conclude that hyperbilirubinemia affects adjoining areas of the brain stem that control hearing and cry production.