Background Pulmonary agenesis is a rare congenital anomaly with a reported prevalence of about 1 in 100,000 births. It may be bilateral or unilateral. Among the unilateral form, left lung agenesis is more common (70%); however, it is the right lung agenesis which carries a dismal prognosis due to the frequent association with a gamut of other congenital anomalies and greater degree of mediastinal shift leading to tracheo-bronchial and vascular distortion. The patients of unilateral pulmonary agenesis usually present in infancy or early childhood. Presentation in late adulthood as seen in our patient is rare. We present a case of left pulmonary agenesis that was diagnosed in 4th decade of life. Case presentation A 36-year-old male presented with gradually progressive exertional dyspnea of 1 month duration. Clinical examination revealed tachycardia and tachypnea. Chest radiograph showed opaque left hemithorax with ipsilateral mediastinal shift. Computed tomography clinched the diagnosis by demonstrating absence of left main bronchus, lung and left pulmonary artery with shift of heart, and great mediastinal vessels into left hemithorax. The patient was managed conservatively and discharged with attachment to out-patient department for regular follow-up. Conclusion Presentation of unilateral lung agenesis in late adulthood, as seen in the present case is extremely rare. This case report highlights that, a rare condition like unilateral pulmonary agenesis, should be considered in the list of differentials in an adult presenting with opaque hemithorax with ipsilateral mediastinal shift on radiography.