BACKGROUND In the modern antibiotic era, Streptococcus agalactiae infection of the endocardium and pericardial space is a rare occurrence. However, once the disease spreads it can lead to life-threatening illness despite advances in diagnostic and treatment modalities, partly because the symptoms and signs associated with pericarditis are frequently missing, and due to the rarity of the disease, diagnosis is often overlooked. We report an extremely rare case of purulent pericarditis caused by Streptococcus agalactiae. CASE REPORT A 65-year-old diabetic woman presented with generalized weakness, high-grade fever, and altered mental status. There were no signs or symptoms suggestive of cardiac tamponade on presentation. A computerized tomography (CT) scan of the chest showed a small pericardial effusion. She was managed for diabetic ketoacidosis and sepsis. An electrocardiogram was significant for new-onset atrial fibrillation. Her clinical status deteriorated rapidly as she developed acute hypoxic respiratory failure and shock. A bedside echocardiogram showed large pericardial effusion around the right ventricle and right ventricular diastolic collapse. She developed cardiac arrest, and during resuscitation bedside pericardiocentesis was done with drainage of 15 cc of serosanguineous fluid. However, the patient could not be revived. Subsequently, blood cultures grew Streptococcus agalactiae a day after she died. On autopsy, she was found to have findings of infective endocarditis and purulent pericarditis. CONCLUSIONS A high index of clinical suspicion is crucial when acute pericarditis is suspected, for early diagnosis and for timely initiation of appropriate therapy with antibiotics and aggressive pericardial drainage to prevent fatal outcome.