We report the details of a young woman in whom pericardial tamponade developed acutely following preparation for allogeneic bone marrow transplantation for acute lymphoblastic leukaemia. The aetiology of the effusion, though uncertain, probably relates to the cumulative cardiotoxicity of cyclophosphamide and irradiation upon a myocardium previously sensitised by anthracycline therapy. As the number of transplant procedures increases, this complication may become more common, and might be avoided by a more critical assessment of cardiac function prior to transplantation, with radionuclide angiography.