Abstract A patient with chronic effusive pericarditis of undetermined etiology, associated with cholesterol crystals in the effusion, is described. A radiocarbon cholesterol turnover study revealed a dominant half-time in the pericardial effusion of sixty days, compared to a fifty day half-time in the serum. The equilibration time of the specific radioactivity of cholesterol in the pericardial fluid with the serum was thirty-three days. A fourteen day difference was noted in achieving a plateau between the serum and pericardial fluid. These findings suggest an abnormality in cholesterol transport. Partial pericardectomy resulted in apparent cure in this case.