A 68-year-old man diagnosed with Erdheim-Chester disease presented to the emergency department with shortness of breath of one-day duration. Upon presentation, the patient was dyspnoeic and hypoxemic. The initial laboratory workup showed raised inflammation markers, and a chest x-ray showed the presence of bilateral lung infiltrates; therefore, he was managed for community-acquired pneumonia with antimicrobial and other supportive measures. Due to lack of improvement, he had transthoracic echocardiography (ECHO), which showed a large pericardial effusion without tamponade. He was treated with corticosteroids and underwent pericardiocentesis, which resulted in remarkable symptomatic improvement. This case presents a serious manifestation of a rare disease and summarizes treatment options from the literature.