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Phenylalanine (4)-hydroxylase (PAH, E.C. 1.14.16.1) is located mainly in liver and converts amino acid phenylalanine (Phe) to tyrosine (Tyr). In 'classical' phenylketonuria (PKU), PAH activity is reduced, whereas in 'atypical' PKU biosynthesis of the cofactor 5,6,7,8-tetrahydrobiopterin (BH4) is disturbed. Aside from these inherited conditions, elevated plasma Phe concentrations and increased Phe to Tyr ratios (Phe/Tyr) were observed in patients with cancer, burns, sepsis and HIV-1 infection. Results indicate that immune activation and inflammation are associated with moderate impairment of PAH activity. This review discusses findings of increased Phe/Tyr in patients suffering from chronic inflammatory diseases, their clinical relevance and consequences.