Two cases of mediastinal emphysema occurring in healthy individuals climbing to the summit of Mount Rainier (4,393 m.) are described. In both, the condition was probably attributable to a voluntary, forced pressure breathing technique--a modified Valsalva maneuver. The various circumstances in which mediastinal emphysema may occur are discussed, along with its possible serious complication of cardiorespiratory compromise. The means of diagnosis are presented, emphasizing that cases in which the important finding of subcutaneous emphysema is absent may be those in which the danger is greatest. Therapeutic measures that occasionally may be necessary are outlined, though the condition is usually a benign one that subsides without specific treatment.