This exploratory pilot study was an open-label, crossover, single-center and non-randomized clinical trial designed to compare the effect of the standardly employed doses of allopurinol (400 mg/day) and febuxostat (80 mg/day) on the urinary 2,8-dihydroxyadenine (DHA) excretion in patients with adenine phosphoribosyltransferase (APRT) deficiency.