Tension pneumothorax is not a rare complication of mechanical ventilation, but its occurrence immediately after intubation with a double-lumen endotracheal tube (DLT) is very rare. Subsequent impairment of the respiratory and cardiovascular function can be life threatening if it is not detected early enough and managed properly. Most reported cases of tension pneumothorax during DLT intubation occurred when one-lung ventilation was applied. Herein we report a case of tension pneumothorax and pneumoperitoneum which occurred immediately after DLT intubation. It appeared before one-lung ventilation was applied because of an inappropriately small-sized and malpositioned DLT.