OBJECTIVE: High noise levels (>70 dB) in the neonatal intensive care unit (NICU) are common in some primary hospitals. This study aimed to investigate the noise in the NICU on auditory system and intelligence development in premature infants. METHODS: One hundred premature infants with respiratory distress syndrome who needed mechanical ventilation therapy were randomly divided into observation and control groups according to the use of earmuffs. The duration of mechanical ventilation therapy lasted for 2 to 15 days in the two groups. After weaning from mechanical ventilator, the auditory brainstem response, cranial B-ultrasonography, and the intelligence development assessment were performed. RESULTS: The percentage of total (23% vs 47%) and mild hearing loss (15% vs 35%) in the observation group was significantly lower than that in the control group (P<0.05) 2 to 3 days after weaning from mechanical ventilator. The incidence of periventricular hemorrhage intraventricular hemorrhage (PVH-IVH) or periventricular leukomalacia (PVL) in the observation group was significantly lower than that in the control group (21% vs 42%; P<0.05). The intelligence development assessment performed in the first 6 and 12 months of life showed that the mental development index and the psychomotor development index in the observation group were much higher than those in the control group (P<0.05). CONCLUSIONS: The noise in the NICU can result in mild hearing loss and retardation of intelligence development and increase the incidence of PVHIVH and PVL in premature infants. The use of earmuff may reduce the adverse events.［Chin J Contemp Pediatr, 2009, 11 (12):976-979］