Tetrahydrobiopterin (BH₄) deficiency is a rare cause of hyperphenylalaninaemia (HPA) and usually leads to progressive neurological deterioration despite early dietary control of plasma phenylalanine concentrations. Dihydropteridine reductase (DHPR) deficiency is the most severe cause with respect to a fatal outcome. We report a 7-year-old girl with HPA diagnosed on neonatal Guthrie screening who at the age of 6 months had cytotoxic therapy for an adrenal neuroblastoma which secreted catecholamines. When 4 years old she was found to have DHPR deficiency. Although developmentally retarded and microcephalic she has failed to develop the florid neurological features often associated with the condition.