Background: Little is known about the consequences of the special energy enriched diet used to treat patients with phenylketonuria (PKU) in terms of obesity and metabolic syndrome (MetSyn) development. Objective: To investigate the prevalence of overweight and obesity, and its consequences in terms of body composition and MetSyn in early treated patients with PKU compared to controls. Design: A sample of 89 patients with PKU (3-30 y; 14.4 +/- 6.6 y) and 79 controls (3-47 y; 16.3 +/- 7.9 y) were studied. In the fasted state, anthropometric, body composition, blood pressure and analytical paratmeters [amino acids, glucose, insulin, total and HDL-cholesterol (HDL-c), triglycerides (TG), high sensitivity c-reactive protein and uric acid] were performed. Data on dietary intake was collected. BMI was classified using WHO criteria, while the definition from International Diabetes Federation (IDF) was used for MetSyn. Results: Prevalence of overweight and obesity (32.6% vs. 24.1%; p=0.293), body fat percentage (22% vs. 23.1%, p=0.581) and central obesity (36.9% vs. 36.4%, p=0.999) were comparable to controls. Patients revealed a higher TG/HDL-c (plt;0.001). The prevalence of MetSyn was 1.5% and 6.1% in patients and controls, respectively. Patients and not controls with central obesity revealed a further significant increase in TG/HDL-c compared with those without central obesity (p=0.023). Conclusion: Patients and controls were similar in terms of overweight and obesity, body composition and MetSyn. However, the dyslipidemia in patients with PKU in relation to overweight and obesity may help us trying to understand the course and the etiology of MetSyn not only in PKU but also in the general population. (C) 2012 Elsevier Inc. All rights reserved.