Objective: Spontaneous pneumomediastinum is uncommon and is traditionally considered a benign, self-limited disorder that usually occurs in young adults without any apparent precipitating factor or disease. The purpose of this study is to present 47 new cases because of their different clinical behaviours. Methods: A descriptive, retrospective case series was conducted to identify adult patients with spontaneous pneumomediastinum who were diagnosed and treated in a single institution between 2000 and 2008. Results: Forty-seven patients were identified, including 33 men and 14 women with a mean age of 27.3 years (range, 16–42 years). Acute-onset chest pain was the predominant symptom at presentation (59.5%); subcutaneous emphysema was the most common sign (42.9%). The most common predisposing factor was consumption of illegal drug (cocaine): it was not a regular and long-term abuse but an occasional trend, coinciding few hours before presenting symptoms (55%). Notably, in one case, a bad outcome with mediastinal shift and important tracheal compression necessitated a right thoracotomy to open the mediastinal pleura. Conclusions: (1) It is known that occasional consumption of cocaine causes diffuse alveolar damage, haemorrhage, etc. The cause–effect relationship consumption–appearance in a few hours of spontaneous pneumomediastinum leads us to appreciate this form of consumption as a causal factor rather than a predisposing/precipitating factor as it appears in the literature on the topic. (2) Spontaneous pneumomediastinum is considered a benign disorder but, as shown by our series, a bad outcome may represent an immediate life-threatening condition, forcing an invasive manoeuvre. We always recommend performing a chest CT scan on admission to predict the evolution.