The treatment of bladder cancer with Bacillus of Calmette-Guerin (BCG) immunotherapy can induce the appearance of a reactive disorder. The Authors describe a 55-year-old male patient with bladder cancer treated with endovesical instillation of BCG immunotherapy, followed after the fifth application by asymmetric oligoarthritis and dactilitis. The observed positivity of both HLA-B27 and HLA-B51 antigens reinforces the hypothesis of a reactive form, possibly through "molecular mimicry" mechanism. The discontinuation of BCG instillation along which a therapeutic attempt with NSAD failed to improve the rheumatic manifestation, which completely remitted after a four-month course of oral steroids. No relapses of joint and tendon involvement was observed during the following five-month period. The clinico-pathogenetic implications suggested by this case are discussed.