Context: Phenylketonuria (PKU) is the most frequent inborn error of metabolism, in which newborns cannot metabolize phenylalanine to tyrosine. Increased phenylalanine in untreated patients with PKU can cause serious intellectual disability; its onerous financial burden also falls on societies. This review study aimed to systematically indicate the frequency of PKU worldwide. We also intended to highlight the global prevalence of PKU, which might shed light on better clinical management and screening programs. 
Methods: In this systematic review, two electronic databases, including PubMed and ScienceDirect were searched for the related literature using relevant keywords: “Phenylketonuria” or “PKU” and “Prevalence” or “Incidence” and “Iran” or “Middle East” or “Europe” or “America” or “Asia.” Accordingly, 4306 reports conducted on PKU from January 2007 to December 2018 were retrieved. With the removal of 44 duplicated publications, 44 reports were included in the current systematic review. Prevalence and incidence rates were categorized based on different continents in which nations used various NBS programs to report the incidence and prevalence of PKU. Non-English, non-eligible, duplicated, animal, and in vitro studies are excluded.
Results: Based on the reported quantitative data, the prevalence of PKU diagnosed worldwide ranged from 0.00044% to 0.02736% in which Italy possessed the highest prevalence; however, Thailand manifested the lowest prevalence rate. However, for some countries, such as India or Finland, either the related data to the frequency of PKU was outdated or overlooked applying any newborn screening programs respecting PKU. 
Conclusions: The current study revealed an elevated prevalence of PKU in Iran, compared with other Asian countries; thus, it demands a more serious management program. Moreover, the high prevalence of PKU in European countries should not be underestimated.