Objectives:  To investigate the effect of diet on total antioxidative status (TAS), transferrin, ferritin and ceruloplasmin serum levels in phenylketonuric (PKU) children. Patients and methods:  Seventeen poorly controlled PKU children underwent clinical and laboratory examinations before, ‘off diet’, and 60 days after adhering to their special diet ‘on diet’, whereas controls (N = 24) were examined once. Blood chemistry was performed with the appropriate methodologies. Results:  Phenylalanine levels differed significantly among the examined groups. Lipids and lipoproteins were higher in ‘off diet’ than in ‘on diet’ group, except of high density lipoprotein and apolipoprotein AI that remained unaffected. Total antioxidative status (386 ± 30 vs 204 ± 23 μmol/L, p < 0.001), ferritin (48.2 ± 2.3 vs 33.0 ± 2.8 μg/L, p < 0.001) and ceruloplasmin (40.02 ± 2.5 vs 25.5 ± 2.8 mg/dL, p < 0.001) levels were significantly higher in ‘on diet’ patients’ group compared to ‘off diet’ one. The low lipoprotein and the high TAS and ferritin levels in patients with PKU ‘on diet’ may be related to the vegetarian diet and the rich in iron formula supplementation. Conclusions:  The low ferritin levels found in ‘off diet’ patients with PKU may be attributed to a decreased liver production of ceruloplasmin, which evaluation may be a useful tool for the follow‐up of patients with PKU.