Treatment of PKU implies for most patients that with strict adherence to dietary treatment they can achieve excellent neurocognitive outcome. Dietary treatment, though, is hard to comply to every day and with every single meal. Unsurprisingly, health-related quality of life (HrQol) is negatively affected if patients have to follow a dietary regime of this kind. Adherence to treatment in PKU is very variable. Factors of significant impact on adherence to treatment and well-being in chronic disease such as self-efficacy or parenting stress have not yet been widely investigated in PKU patients. The ideal treatment prescription (and guideline) recommends as much as necessary and as little as possible, based on the best evidence available. Patients should neither be deprived of treatment options nor be exposed to overtreatment. This study investigates adherence, metabolic control, HrQol in PKU patients treated by centres which follow different guidelines