A 74-year-old female has been diagnosed with stage IIIB rectal cancer in 2003. Following anterior resection, she received adjuvant chemotherapy with three different regimens. In August 2005, she was started on a modified FOLFOX6 regimen, and the sixth cycle of chemotherapy induced a severe hypersensitivity reaction (HSR). Immediate cessation of the infusion resulted in a disappearance of the allergic reaction 60 min later. Blood tests just after the reaction demonstrated a marked elevation of immunoglobulin E to 300 IU L(-1) (normal range: <170 IU L(-1)). This change implies the involvement of a type I reaction in the HSR. In addition, a drug lymphocyte stimulating test against oxaliplatin and levofolinate calcium (an isomer of leucovorin calcium) gave values of 696% and 107 % respectively, as compared with control serum. This suggests that the patient had an adverse reaction not only of type I but partly of type IV allergic reaction also. Oxaliplatin appears to have caused a HSR in this Japanese patient, and thus pharmacists, physicians, and other medical staff must keep a careful watch of a patient's clinical condition during chemotherapy including oxaliplatin.