BACKGROUND Cardiac tamponade caused by pericardial effusion has a high mortality rate; thus, it is important to diagnose and treat this condition immediately. Specifically, bacterial pericarditis, although now very rare, is often fatal because of its fulminant process. CASE REPORT We present a case of a 61-year-old man with metastatic small cell lung cancer undergoing chemotherapy who presented with fatigue, poor appetite, and altered mental status. He was found to have a large-volume pericardial effusion with tamponade physiology. He underwent emergent pericardiocentesis. The pericardial effusion was nonmalignant, with cultures growing Streptococcus pneumoniae. It was only after his emergent pericardiocentesis that previous imaging from one month prior was able to be reviewed, which showed possible right upper lobe abscess. CONCLUSIONS Most pericardial effusions in cancer patients are related to their malignancy, either due to direct metastasis or secondary physiologic effects. This case is a unique example of a lung cancer patient presenting with a pneumococcal pericardial effusion, which in itself is a rare phenomenon. This case report demonstrates the importance of considering early antibiotic therapy in patients presenting with pericardial effusion, especially given the high mortality rates of infectious pericardial effusions.