A very rare case is being described; common pleural cavity was accidentally diagnosed in a 3-year-old boy operated for funnel chest (pectus excavatum). During 36 years 516 patients were operated in our department and we often notice pectus excavatum associated with other types of congenital pathology but only one had the common pleural space. In normal human beings pleural space is divided into left and right chambers separated by the mediastinum with no communication in between. In some mammals such as pigs, cows etc. a congenital communication is found between the pleural cavities, but this type of communication is very rare in humans and most often is of acquired origin. Pleural communication may also develop after major cardiothoracic surgery. In this case a 3-year-old male patient was admitted for the elective surgery on pectus excavatum. Clinical examination showed a very deep funnel chest. Both the heart and the mediastinum are left-shifted by the deformed breastbone; it is clearly demonstrated on a plain and lateral X-ray. On the left, beside the main vessels, an indistinct patch is noted. Typical M. Ravitch procedure was performed, by accident the pleural space was opened. Both pleural cavities had an evident communication along the anterior mediastinum. The torn pleura was sutured, the excess air removed by a puncture. Postoperative period was uneventful, additional treatment was not needed; currently the boy is feeling well. The postoperative X-ray showed the heart and the mediastinum to return to normal position.