Phenylketonuria (PKU) is an inborn error of metabolism characterized by pathogenic variants of the phenylalanine hydroxylase (PAH) gene with a resulting accumulation of phenylalanine (Phe) to neurotoxic levels. Diagnosis of PKU in the Latvian population began in 1985 and the present study’s aim was to evaluate the available data on all PKU patients in Latvia. Phe concentrations were measured in dried blood spots using a Neonatal Phenylalanine kit (Labsystems Oy, Finland). Genomic DNA was analyzed for pathogenic variants in the PAH gene. Biochemical data were available through follow-up visits of the 83 patients. In 97% of patients (99 of 102), pathogenic variants were detected on both alleles. With an occurrence of 69.6%%, the most common pathogenic variant was the severe pathogenic variant p. Arg408Trp. The available data for 83 patients revealed that metabolic control was better in younger age groups and worse in adults.