Anterior chest wall and anterior mediastinal infections are very rare in children with no predisposing thoracic surgery. Congenital sternal defects occur as a result of incomplete ossification of the sternal bone, and show a wide range of severity. We present the case of a 15-month-old boy who presented to the A&E department with an anterior chest-wall mass. After investigation, it was found to be an infective mass, with extension into the anterior mediastinum, through a previously undiagnosed 5 mm congenital sternal defect identified peri-operatively. After antibiotic therapy and surgical management the patient made complete recovery with no recurrence or complications within 6 months of review.