Of 1041 renal allograft transplantations performed in our centre, 142 (13.6%) were carried out with ABO-minor-incompatible kidneys. Anti-recipient ABO antibodies were found in two of 34 patients treated with cyclosporin. These two cases of severe but self-limited haemolysis due to anti-A1 and anti-B, respectively, are reported in detail. Among 108 azathioprine-treated patients no evidence of the disorder was found. The condition should be suspected in any recipient with an unexpected reduction in haematrocrit or other signs of haemolysis after ABO-minor-incompatible organ transplantation.