Phenylketonuria is an inborn error of metabolism of autosomal recessive inheritance, with partial or total deficiency of the phenylalanine hydroxylase hepatic enzyme, which converts L-phenylalanine into tyrosine, thus causing accumulation of phenylalanine at the brain and serum level, interfering in the brain protein synthesis and entailing serious deficits. The objective of this study was to analyze the effectiveness of Phenylketonuria Diagnosis in the neonatal treatment reference service. This is a cross-sectional, analytical study with a quantitative approach, documented with retrospective data collection. Sociodemographic data, diagnosis, onset of treatment and the coverage rate analysis were grouped on a quadrennial basis. The sample consisted of 14 patients, from whom 57.1% had records of birth and collection time. In variable days of life, 28.6% were screened within the recommended period, 71.4% were diagnosed up to one month of life and 1 case at 3 years of age, for the onset of treatment (14.3%). The ideal collection would be performed up to 30 days of life. The lowest coverage rate for quadrennial was between 2014-2017 with 84.3%, with an incidence of 1:21,933. In conclusion, we highlight the need to optimize the neonatal screening service in order to make early diagnosis, begin specific treatment and minimize or eradicate irreversible sequelae.