The pH of pericardial fluid has not been well characterized. Therefore, it was analyzed in 13 consecutive patients who had pericardial disease of varying causes and were undergoing pericardiocentesis. Values ranged from 6.82 to 7.59. In seven patients with bloody pericardial effusions, the pH was significantly lower than the simultaneously determined arterial pH (7.31 +/- 0.07 versus 7.45 +/- 0.04, p less than 0.01). Pericardial fluid pH discriminated between inflammatory (pH 7.06 +/- 0.07) and noninflammatory (pH 7.42 +/- 0.06) causes (p less than 0.001). Thus, pericardial fluid pH determination is a quick and accurate method to exclude inadvertent ventricular punctures in bloody pericardiocentesis and to distinguish between inflammatory and noninflammatory causes of pericardial fluid accumulation.