We report a patient who relapsed in a patella and knee joint after allogeneic bone marrow transplantation (BMT) for Ph chromosome-positive acute lymphoblastic leukemia. The patient complained of pain and swelling of knee joint 14 months post-BMT. Fluid from the knee joint included leukemic cells consistent with the immunophenotype of blasts prior to BMT and also revealed the bcr/abl transcript by reverse-transcriptase polymerase chain reaction. Magnetic resonance imaging demonstrated an abnormal signal in the patella. Radiotherapy to the localized extramedullary lesion was successful and no bone marrow relapse has been detected cytologically and cytogenetically to date. This case suggests that the physician should be aware of unusual relapse sites of leukemia post-BMT.