Intravesical instillation of BCG (Bacille Calmette-Guerin) is a therapy prepared with weakened strains of Mycobacterium bovis and is an effective complementary treatment for high-risk (non-musculoinfiltrating or non-invasive) bladder cancer. Although its safety for clinical use is high, endovesical immunotherapy is not without complications. Arthralgia and arthritis are infrequent, but potentially severe, complications, the early diagnosis of which can allow adequate medical treatment and avoid chronification of the pathology. This case shows the therapeutic management of hip osteoarthritis in a 59-year-old woman, a rare and serious complication, secondary to a rare pathology such as reactive arthritis due to BCG instillation.