Introduction: Spontaneous pneumomediastinum (SPM) is rare, with an incidence of 1/25,000. It is defined as extra luminal free air within the mediastinum, not associated with trauma. A classic clinical triad consists of pleuritic chest pain, dyspnoea and subcutaneous emphysema. SPM is self-limiting and symptoms can be managed conservatively. However despite a good prognosis, secondary causes should be excluded. Case description: Miss AG, 27 year old with a known history of Ulcerative Colitis being treated with azathioprine and adalimumab, presented to the ED with a two week history of progressively worsening shortness of breath and left sided pleuritic chest pain. She reported a 3 day h/o ongoing fever, rigors for which she was being treated with Amoxicillin 500mg tds. The patient had no previous history of respiratory conditions and was a non-smoker. The patient saturated at 96% on 2L of Oxygen with a respiratory rate of 19. She was apyrexial, normotensive and acyanotic. Clinical findings on auscultation revealed left sided bronchial breathing, an erect postero-anterior chest x-ray revealed dense opacification throughout the left mid and lower zone. As she was increasingly symptomatic, she went onto have a CTPA, which showed a pneumomediastinum. A discussion with the thoracic surgeons followed and an urgent CT thorax and abdomen with oral gastrografin was carried out to exclude oesophageal perforation. This scan did not rreveal any extravasation of contrast around the oesophagus to suggest a perforation or a leak. The patient improved clinically, discharged in 6 days, she was kept nil by mouth for 48 hours once the CT had confirmed that there was no perforation. Results and Conclusions: SPM usually has a benign and favourable clinical course and is usually self-limiting. It is more commonly seen in young men. The pathophysiological process behind SPM was initially described by Macklin in 1944, who described a rupture of the terminal alveoli, secondary to pressure differences across the alveolar membrane causing air to leak into the lung interstitium and consequently into them mediastinum. In a retrospective study by Park et al., 44% of the patients had a precipitating factor, with the most common being cough. Literature suggests that the most common symptoms repported in cases are usually chest pain, which is usually pleuritic in nature and dyspnoea Subcutaneous emphysema is also reported as common clinical sign and has a frequency ranging from 40% to 100% in cases. Moreover, the characteristic sign of systolic crackles on auscultation known as Hamman’s sign can also be heard in 30% of cases. Diagnosis in this case was made based on the CTPA though it is more commonly based on chestx-ray findings. Postero-anterior view establishes the diagnosis in about two-thirds of patients with the three commonest findings: air streaks in the superior mediastinum, prominent left sided silhouette of the heart and subcutaneous emphysema of the neck and shoulder. For this reason, a chest CT scan is a more conclusive and sensitive scan, which is considered the gold standard investigation for SPM. More importantly, a CT scan with an oral contrast, as in this case, will allow us to distinguish between secondary causes of a SPM such as an oesophageal leak or rupture. The prognosis is good and treatment is mainly conservative for those diagnosed. The average clinical course, reported in the retrospective study by Takada et al. was 1.8 days after diagnosis, with an average of 7.8 day of hospitalization. Complications such as hypertensive pneumomediastinum, pneumopericardium, and mediastinitis can occur and patients should be monitored throughout their admission. Take home message: In conclusion, spontaneous pneumomediastinum is a rare condition, and is known to have a favourable clinical course, with symptoms of pleuritic chest pain and dyspnoea improving within a few days. Chest CT is the gold standard radiological investigation to confirm diagnosis whereby measures must be taken to exclude secondary causes of SPM. Patients can be treated conservatively and must be monitored for life threatening complications.