The aim of this study was to determine whether any relationship exists between the severity of mutation of the phenylalanine hydroxylase (PAH) gene and the plasma concentrations of phenylalanine (Phe) and tyrosine (Tyr) under fasting and semifasting conditions among heterozygotes in a matched case-control study. Parents of patients affected by PAH deficiency (n = 25) detected through the Italian Neonatal Screening Program and referred from January 1994 to June 2000, and parents of healthy children were investigated. In total, 68 subjects without any disease, 34 hyperphenylalaninaemia (HPA) heterozygous parents and 34 age- and gender-matched controls, were recruited. Plasma concentrations of Phe and Tyr in fasting and semifasting (1600 mg Phe oral load) conditions were the main outcome measures. DNA analysis for PAH mutations was performed in all 68 subjects. Compared with controls, heterozygotes showed higher fasting and semifasting Phe concentrations (p < 0.0001), lower semifasting Tyr concentrations (p = 0.015), lower Tyr variations (p = 0.003) and a higher Phe/Tyr ratio (p < 0.0001) in switching from fasting to semifasting conditions. Heterozygotes carrying a severe mutation showed semifasting plasma Tyr concentrations lower than controls (p = 0.019) but not significantly different from Tyr levels found in non-severe carriers (p = 0.197). The Tyr variations were minor in severe carriers than controls (p < 0.001) and non-severe carriers too, although with lower significance (p = 0.089). In six carriers of A403V mutation, parents of mild hyperphenylalaninaemics on an unrestricted diet, significant differences in variations from fasting to semifasting conditions were found compared with parents of patients on a diet.Although the great heterogeneity of PAH mutations limits any general conclusion, the results suggest that monitoring plasma Tyr variations may be more sensitive than plasma Phe in assessing the severity of PAH mutations in HPA heterozygotes.