Six of 9 patients with previously documented adverse reactions to allopurinol exhibited transformation of their peripheral blood lymphocytes when these were exposed in vitro to the allopurinol metabolite, oxypurinol. In 2 of these subjects, intradermal skin challenge was positive at 48 hours with either allopurinol or oxypurinol. The evidence presented suggests that some adverse reactions to allopurinol represent delayed-type hypersensitivity to oxypurinol.