Pericardial hydatid cyst is rare. We present a 32-year-old man with a pericardial. hydatid cyst. Chest x-ray film showed a spherical mass located left mediastinum. A pericardial cyst with low signal intensity was noted on T1 weighted magnetic resonance (MR) images. The cyst had high signal intensity, but signal intensity of folded parasitic membranes in the cyst were seen lower intensity on T2 weighted MR images. Based on these MR findings, pericardial hydatid cyst was diagnosed. The patient underwent surgical removal of the cyst. Histologic study of the cyst confirmed hydatid cyst diagnosis. (C) 2003 Tohoku University Medical Press.