Pulmonary barotrauma (PBT) usually develops in Self Contained Underwater Breathing Apparatus (SCUBA) divers if there is air trapping during ascent caused by uncontrolled decompression, breath holding and emerging of air trapping in the lungs. The air expanded in lungs according to the Boyle's Law should be exhaled enough during ascent to prevent PBT. A 36 years-old-male diver made breath-hold diving to 9 meters. He breathed compressed air from the equipment of a SCUBA diver at the same depth. Afterwards he surfaced leaving the SCUBA diver and did not exhale during ascent. Some air came out of his lung involuntarily before reaching the surface. He had chest pain, dysarthria and sensory changes at surface. Although his neurological complaints disappeared within 30 minutes, substernal pain persisted. The chest computed tomography (CT) scan revealed mediastinal emphysema that disappeared in 3 weeks without treatment. In this case, the diver neglected to exhale the expanded air in the lung during ascent.