Unilateral absent pulmonary artery (UAPA) is a congenital abnormality rarely diagnosed in adults. UAPA has a myriad of clinical presentations and pulmonary hypertension is present in a quarter of all cases. Isolated UAPA commonly affects the right pulmonary artery and occurs as a result of abnormal development of the sixth aortic arch segment. Due to its rarity, it remains a diagnostic and therapeutic challenge. We describe a case of UAPA in an adult presenting with severe pulmonary hypertension. We describe the appropriate diagnostic approach to a patient with pulmonary hypertension and illustrate the importance of a detailed evaluation to determine the underlying aetiology, particularly in rare causes. Furthermore, we review the clinical presentation, diagnosis and management challenges of UAPA in adults.