Congenital diaphragmatic hernia consists of a defect of the diaphragm that allows the passage of abdominal viscera into the thorax. Congenital diaphragmatic hernia most commonly present in infancy and childhood due to respiratory distress. However, a few of them may escape into adulthood. A very few of these adult cases may be identified during a routine medical checkup for various other reasons without any cardio-respiratory problems. In our case report, we have diagnosed incidentally an asymptomatic left diaphragmatic hernia with cardio-mediastinal shifting in a 38 years old female admitted for left breast fibroadenoma excision during a routine pre-anesthetic check-up and underwent breast surgery without any cardio-respiratory problems.