Published work suggests that cardiac tamponade occurs only occasionally after bone-marrow transplantation (BMT) but the worrying number of cases encountered in the transplant programme in Pesaro, Italy, has led to an analysis of this complication. Cardiac tamponade occurred in 8 (2%) of 400 consecutive thalassaemic patients during conditioning for or within a month of BMT. 6 cases were fatal; these represented 9% of all causes of death and 29% of those occurring between start of conditioning regimen and 30 days post transplant. The syndrome was characterised by sudden onset of circulatory shock and cardiac arrest. The only effective treatment was immediate fluid removal. The absence of myocardial lesions and the complete resolution of the syndrome after pericardiocentesis suggest that the pericardial membranes played the main part in the pathogenesis of the syndrome. Since irradiation was not part of the conditioning regimen and since 3 of the affected patients had bacteraemia, the triggering factor for the syndrome could have been the drugs used for conditioning, acting alone or together with bacteraemia and trauma. The frequency with which we encountered the syndrome, and the similarity among our patients in clinical picture, and in characteristics of the effusion, indicate that cardiac tamponade occurring in thalassaemic patients after start of chemotherapy as conditioning for BMT is a specific syndrome requiring rapid treatment.