Sensitisation to HLA antigens, formed as a result of transfusion, previous allografts or pregnancy, remains a significant problem in transplantation. The aim of this study was to define the causes of sensitisation of potential renal allograft recipients in Ireland in the post-EPO era. A retrospective survey of all patients who were active on the renal transplant waiting list during 1996 was performed evaluating the panel reactive antibodies (PRA), history of sensitising events, and waiting time for transplantation. Of a total of 244 patients (151 males, 93 females), 163 (67%) were not sensitised (PRA < 10%), and 35 (14%) were sensitised (PRA 10-59%), 15 subjects (6%) were significantly sensitised (60-79%) and 31 (13%) were highly sensitised (PRA > 80%). Only 59 (24%) patients had no sensitising events recorded. Fifty-eight (24%) subjects had previous allografts. Transfusion was documented in 173 (71%) subjects. Pregnancy was noted in 55/93 (59%) female subjects. All highly sensitised patients had been transfused. Eighty per cent (12/15) significantly sensitised and 97 (60%) of the non-sensitised subjects had been transfused. The level of sensitisation clearly increased with the number of units transfused. Non-sensitised subjects received a mean of 5.65 units (SEM 1.38), sensitised subjects a mean of 9.8 units (SEM 3.17), significantly sensitised a men of 18.2 units (SEM 6.51), while highly sensitised subjects received a mean of 37.8 units, (SEM 8.4). There was a direct relationship between the waiting time for transplantation and the degree of sensitisation. The percentage of patients who had been transfused in the 1996 cohort (71%) was similar to the percentage of patients transfused on the waiting list in August 1999 (75%). These data demonstrate that transfusion remains an important cause of sensitisation, despite the use of EPO. Measures to further reduce the use of transfusion or the use of immunosuppression at the time of transfusion in potential allograft recipients may be of value in the future.