The purpose of this study was to determine the prevalence of developmental defects of enamel in patients diagnosed with phenylketonuria (PKU).The study group consisted of 24 four- to 24-year-old subjects with PKU. The control group consisted of 24 healthy individuals. An examination for the detection of developmental defects of enamel was conducted at the university pediatric dentistry clinic by a single examiner. Data were analyzed using the chi-square test (P<.05) and odds ratios.The prevalence of developmental defects of enamel was 36 percent in the study group and 15 percent in the control group. The maxillary central incisors were the most affected teeth in patients with PKU, while the maxillary and mandibular first molars were the most affected teeth in the control group. Patients with PKU had a 3.3-fold greater chance of exhibiting developmental defects of enamel versus the healthy controls, which was statistically significant (P<.001).The study findings suggest that PKU increases the risk of developmental defects of enamel.