A 40-year-old man with pain in his left, swollen knee that persisted for 6 weeks presented with chest pain, dyspnoea and subfebrile temperature. The pain worsened during inspiration and was relieved by sitting up straight. The electrocardiogram showed pericarditis. The patient was treated with high-dose carbasalate calcium. Initially, echocardiography revealed a 2-cm pericardial effusion with no signs of influx inhibition. Blood cultures were positive for Neisseria meningitidis, and treatment was expanded to include antibiotics. Based on a deterioration in patient condition and the tamponade image, pericardiocentesis was performed. Repeated transoesophageal echocardiography showed insufficient drainage of the purulent pericardial effusion. Pericardiectomy was then performed. The patient was doing very well, 3 years after this. If left untreated, the mortality rate for purulent pericarditis approaches 100%. It is therefore important to diagnose at an early stage so that treatment with antibiotics and surgery, which can reduce mortality considerably, can be performed.