Background: Aortopulmonary window (APW) is a rare congenital heart defect, accounting for only 0.2–0.6% of all congenital cardiac anomalies, and is usually identified and surgically corrected within the first year of life to prevent irreversible pulmonary hypertension (PH). Once pulmonary vascular resistance (PVR) becomes fixed, surgical repair is generally considered contraindicated. However, emerging evidence suggests that pulmonary vascular reactivity may persist in select adolescents, opening a potential therapeutic window beyond infancy. Case presentation: We report a 15-year-old male with a large APW diagnosed in infancy but lost to follow-up, who presented in adolescence with exertional dyspnea. Initial cardiac catheterization revealed a high indexed PVR (10.89 WU·m²) and absent oxygen vasoreactivity, indicating inoperability. Despite this, preserved biventricular function and lack of cyanosis prompted a trial of targeted medical therapy with sildenafil, spironolactone, lisinopril, and digoxin. After 12 months, repeat catheterization showed dramatic haemodynamic improvement: baseline PVRi decreased to 8.84 WU·m² and fell further to 0.76 WU·m² after 100% oxygen, with a reduction in PVR/SVR ratio from 0.58 to 0.04. The calculated Qp/Qs increased to 16.77, although this extreme value was considered likely overestimated because systemic and pulmonary arterial oxygen saturations were almost identical. Definitive surgical repair was undertaken with excellent early results. On follow-up, the patient remained asymptomatic with preserved biventricular function, low estimated pulmonary artery pressures and no residual APW on echocardiography. Conclusion: This case suggests that operability in APW may not be irrevocably lost beyond infancy in carefully selected adolescents. It underscores the importance of individualized assessment, medical preconditioning and serial haemodynamic reassessment to unmask latent pulmonary vasoreactivity. Larger series are needed to define which late-presenting patients with APW and PH may safely benefit from definitive surgical repair. © The Author(s) 2026.