Allopurinol is a xanthine oxidase inhibitor widely used to control plasma uric acid levels. Episodes of hypersensitivity to the drug are not rare. A severe form of this with a generalized exanthem, fever and liver involvement has been termed the allopurinol hypersensitivity syndrome (AHS). Patch testing and lymphocyte stimulation testing (LST) are not helpful in confirming this sensitivity. Allopurinol works as a substrate of xanthine oxidase, and is rapidly oxidized into oxypurinol in vivo. Therefore, the biological half-life of oxypurinol is markedly longer than that of allopurinol. In addition, conspicuous pre-existing renal impairment has been noted in many AHS patients. Thus, it is possible that AHS is a manifestation of hypersensitivity to oxy-, not allopurinol. Here, we now report three cases of AHS in which there were significant lymphoproliferative reactions to oxypurinol but not allopurinol.