PURPOSE: To investigate the existence of hearing impairments in infants with phenylketonuria with early diagnose and treatment, and to compare the audiological findings with those of their normal peers. METHODS: Vocal and pure-tone audiometry and acoustic immitance tests were conducted in 63 children, 30 from a control group, with mean age of 8.1 years, and 33 from a study group, with phenylketonuria and mean age of 7.7 years. The study group was subdivided according to diet control: 15 subjects had adequate (mean age of 8.1 years) and 18 had inadequate diet control (mean age 7.2 years). Statistical analysis used t-test or ANOVA. RESULTS: Audiometry showed that, in the control group, 83.3% of the subjects had normal hearing, and 16.7% had uni- or bilateral conductive hearing loss. In the study group, 66.7% of the subjects presented normal hearing, and 33.3% had conductive hearing loss. Immitance measures showed normal results in 91.7% of the children from the control group, and in 72.7% of the subjects with phenylketonuria. Differences were found between the groups regarding hearing thresholds, acoustic reflexes, speech reception thresholds, and speech recognition. No differences were observed between the results of phenylketonuria subjects with adequate and inadequate diets. CONCLUSION: Children with phenylketonuria early diagnosed and treated presented worse conductive hearing thresholds, speech reception threshold and speech recognition when compared to their normal peers, as evidenced in pure-tone and vocal audiometry.