OBJECTIVES: Hypothermic machine perfusion preservation has been shown toimprove the outcome of renal grafts from non-heartbeating donors. We have noticed that organs gain avariable amount of weight during perfusion. METHODS: All grafts, whether used or discarded, were assessedwith respect to their weight gain. Primary outcome measures were the results of viability testing. Secondaryoutcomes were early transplant variables including incidence and duration of delayed graft function andhistological examination of protocol graft biopsies. RESULTS: Weight increase data was available on 97grafts. There were no significant differences in weight gain between kidneys used and discarded (17%vs 20%). 14 patients received grafts which gained over 30% of their initial retrieval weight. There wereno significant differences in secondary outcome measures between this group of recipients and patientswho had received less "waterlogged" kidneys. Histological changes including endothelial inflammationand oedema were observed. CONCLUSIONS: Kidney grafts which have gained over 30% of weight on hypothermicmachine perfusion preservation can be transplanted successfully. Complex interactions between the periodof warm ischaemia, in situ flushing, perfusion pressures and perfusate probably lead to the phenomenonof excessive graft weight gain.