To understand the effects of hyperphenylalaninaemia on fetal growth, we studied growth parameters (weight, length and head circumference) of 23 phenylketonuric (PKU) and 60 hyperphenylalaninaemic (HPA) newborns from healthy mothers and of 1853 healthy neonates from north-east Italy. A comparison of the growth parameters for both PKU and HPA newborns, as well as for controls, showed a statistically significant higher percentage of PKU and HPA patients with reduced body length and cranial circumference (P < 0.05 for both parameters in affected neonates). The z-scores for all growth parameters regarding both PKU and HPA newborns and controls, and between PKU and HPA newborns according to the Mann-Whitney non-parametric test, were statistically significantly lower in PKU newborns than in controls; in contrast, only body length was significantly lower in HPA newborns than in controls (P < 0.01). A comparison of growth parameter z-scores using the Kruskal-Wallis test for PKU, HPA and control newborns showed that both body length (P < 0.01) and cranial circumference (P < 0.05) were significantly lower in both groups of affected neonates. Our results showed intrauterine growth retardation for both PKU and HPA newborns. Body length and cranial circumference appeared to be more important than birthweight in evaluating growth of PKU and HPA newborns.