Pulmonary agenesis is a rare congenital lung defect involving the complete absence of the primary bronchus as well as pulmonary parenchyma and vessels. Unilateral agenesis occurs at the rate of 1/10,000–20,000 pregnancies. The cause of this defect is still unknown. This abnormality does not cause any specific symptoms; it may lead to respiratory failure, but an asymptomatic course lasting many years is also possible. This defect can often be suspected based on a chest radiograph performed for other reasons. We present the case of a 15-year-old boy with an accidental diagnosis of left pulmonary agenesis. Upon diagnosis the patient did not report any complaints. During a routine visit at an allergy clinic decreased vesicular sounds were present on the left side. A subsequent chest radiograph was reported as showing left lung inferior lobe atelectasis, which was an indication for a more comprehensive diagnostic investigation. A computed tomography scan revealed the complete absence of the left lung.