Summary Over 1000 kidney transplant patients were tested for cytotoxic antibodies before transplantation. It was found that patients with preformed antibodies had a significantly poorer outcome than those without antibodies in terms of clinical ranks and survival. This effect was over and above the instances of hyperacute failures previously shown to be associated with preformed cytotoxins. Among patients who received second transplants from cadaver donors, an extremely high failure-rate was observed in patients who had developed antibodies following the first graft, whereas if antibodies were not present, the failure-rate was comparable with that of first transplants done in patients without antibodies. By analysis of survival curves using logarithmic plots, it is postulated that pre-immunization has its greatest effect in the early 3-6 month high risk period and magnifies incompatibilities which occur with unrelated cadaver donors.