OBJECTIVE--To establish criteria for the accurate diagnosis of different forms of left sided pericardial defects on magnetic resonance imaging. Early detection of a partial apical defect is essential as it is potentially fatal. DESIGN--Examination of four children with congenital pericardial defects by magnetic resonance imaging, the results being compared with the features on conventional chest radiography and echocardiography and with published data. RESULTS--Magnetic resonance imaging improved the ability to diagnose and distinguish between complete and partial left sided pericardial defects. A deep myocardial crease was visualised in a patient with apical pericardial defect, indicating the risk of a life threatening ventricular strangulation. A prominent left atrial appendage was, in contrast to many reports, not a reliable sign for partial left sided pericardial defect. CONCLUSIONS--The various forms of congenital left sided pericardial defects cannot reliably be diagnosed in plain chest radiographs or on echocardiography. Their diagnosis and the distinction between partial and complete defects, however, is of clinical importance and can be accomplished more confidently by magnetic resonance imaging.