To assess whether growth retardation in patients with phenylketonuria (PKU) is related to the strictness of their dietary treatment, the relationship between Z scores for height up to 3 y of age and different indices of dietary control in 103 early treated Dutch PKU patients was studied. As indices of dietary control, the mean phenylalanine (Phe) concentration, the frequency of plasma Phe concentrations < 200 and < 120 micromol/l, and the standard deviation of the individual plasma Phe concentrations were studied. These measures of the dietary control were divided into quartiles. The mean Z score of the studied patients showed a decrease of 0.18/y (SD 0.36). No statistically significant difference between any of the quartiles of the studied indices with growth retardation was found. None of the used indices of dietary control based on plasma Phe concentrations showed a relationship between different degrees of the strictness of dietary treatment with growth retardation in Dutch PKU patients.