This observational cross-sectional multicenter study aims to assess the prevalence of overweight and obesity in adult patients with phenylketonuria (PKU) and to identify associated risk factors. Adult PKU patients followed in metabolic centers in Switzerland and Austria will be evaluated using clinical data, laboratory parameters, and standardized questionnaires addressing diet, physical activity, and psychological factors. The study seeks to better understand contributors to weight gain in this population to inform preventive strategies.