We reviewed the clinical and laboratory features of six patients with pericarditis caused by Streptococcus pneumoniae who were admitted to Boston City Hospital. The diagnosis of pneumococcal pericarditis was delayed or missed entirely during life in all patients. The frequent absence of pericardial friction rubs and cardimegaly on chest roentgenograms contributed to the difficulty in recognizing this illness. Electrocardiograms and physical examinations of the heart almost always disclosed abnormalities, but they were not sufficiently specific to suggest pericarditis, and patients were often thought to have had an acute complication of arteriosclerotic heart disease. Review of the English literature since 1945 supports the recent experience in our hospital that the diagnosis of pneumococcal pericarditis may be elusive.