Aim: To evaluate whether the age at body mass index (BMI) rebound may be associated with overweight at age 8 y in hyperphenylalaninaemic (HPA) children. Methods: A longitudinal observational study including 97 HPA children born 1984–1993 and detected by the National Neonatal Screening programme. Children were followed up at the same institution and evaluated for dietary intakes and anthropometrical parameters from diagnosis up to the age of 8 y. Outcome measure was overweight at age 8 y, defined according to the International Obesity Task Force. The age at BMI rebound, BMI before and at rebound were considered as potential determinants. Familial overweight, breastfeeding and macronutrients intake at age 1 y were considered as confounders. Results: Mean (95% confidence interval [CI]) age at BMI rebound was 5.0 (4.7–5.3) y. At the age of 8 y, 24.7% (95% CI 16.3–33.1%) of children was overweight. Children overweight at the age of 8y exhibited earlier BMI rebound than non‐overweight children (mean difference [95% CI] —2.1 [‐2.8 to ‐1.4] y) and higher BMI from the age of 1 y (mean difference [95% CI] 1.2 [0.9–2.5] kg/m2) onward. Overweight was more likely in children with, rather than without, parental overweight (41.0% vs 19.8%). After adjustment for confounders, logistic analysis showed that earlier BMI rebound (odds ratio [OR] 2.4, 95% CI 1.2–4.8) and BMI at age 1 y (OR 2.3, 95%CI 1.1–4.98) were independently associated with overweight at the age of 8 y. Conclusion: Within the population of this study, overweight at age 8 y was positively associated with early BMI rebound and BMI at age 1 y.