ABO-incompatible kidney transplantation (ABOi-KT) broadens transplantation options in situations with a shortage of donors. After immediate transplantation, isoagglutinin titer rebound can cause acute antibody-mediated rejection (AMR), but several weeks later, even if the isoagglutinin titer increases, it is considered that there is no graft damage due to accommodation. Here we present three cases of late AMR and suggest prolonged isoagglutinin titer monitoring. A 61-year-old woman underwent ABOi-KT (recipient: B, donor: A). On postoperative day (POD) 6, she underwent small bowel resection due to panperitonitis. On POD 17, creatinine (Cr) levels began to rise along with the onset of fever. On POD 19, her isoagglutinin titer was increased (1:64), and continuous renal replacement therapy was initiated. On POD 27, graftectomy was performed. The second patient was a 52-year-old woman (recipient: B, donor: A). On POD 6, she underwent therapeutic plasma exchange (TPE) due to increased isoagglutinin titer (1:16). On POD 11, C-reactive protein (CRP), Cr levels, and isoagglutinin titer (1:32) increased, with onset of fever. She started daily TPE. On POD 20, computed tomography revealed graft necrosis, resulting in graftectomy on POD 22. The third patient was a 49-year-old male (recipient: O, donor: A). On POD 13, the patient had fever, reduced urine output, and increased Cr and CRP levels. On POD 14, Isoagglutinin titer (1:16) was elevated. On POD 34, Cr, isoagglutinin titer, and CRP started to rise, along with decreased urine output. The patient is now undergoing immunosuppressive treatment including TPE. The Korean National Medical Insurance guarantees ABO antibody titer testing up to 2 weeks after ABOi transplantation. However, as demonstrated in the above cases, there could be an elevation in isoagglutinin titer after 2 weeks leading to graft damage, often accompanied by increase in CRP. Long-term monitoring isoagglutinin titers should be possible when symptoms or laboratory findings related to infection are present, to prevent poor prognosis.