During the past three years, more than 500 patients were seen with mediastinal or cervical emphysema or both the Ben Taub General Hospital Emergency Center. In all but 15 there was a history of recent trauma or antecedent respiratory conditions with repeated sustained intrabronchial pressure, such as asthma. These 15 patients had a common history of repeated sustained Valsalva's maneuvers during marijuana smoking or during intravenous injections of heroin. The results of esophagography, bronchoscopy, and esophagoscopy were normal in these 15 patients. Conservative management consisted of close observation and re-evaluation for possible causes of secondary pneumomediastinum. All recovered without sequelae with resolution of the pneumomediastinum within 2 to 5 days. Radiographic and physical findings in primary pneumomediastinum resulting from prolonged intraalveolar hyperpressure are similar to those seen in patients with trauma or spontaneous rupture of the esophagus.