Background: Immunotherapy with monoclonal antibody 17-1A (mAb 17-1A) has been shown effective as an adjuvant treatment in UICC stage III colon carcinoma. Usually, severe side effects are infrequent with mAb 17-1A treatment. Case Report: A 64-year-old man had a 18-month history of recurring arthralgia, sinusitis, and conjunctivits. After curative resection of UICC stage II colon cancer adjuvant treatment with mAb 17-1A was initiated. After the first administration (500 mg) the patient experienced an aggravation of the above-mentioned symptoms which led to the diagnosis of Wegener’s granulomatosis with multiorgan involvement. Under immunosuppressive therapy with cyclophosphamide and prednisone, clinical stabilization could be achieved. Conclusion: The exacerbation of Wegener’s granulomatosis occurred immediately after the first administration of mAb 17-1A. This suggests that mAb 17-1A should be applied cautiously in autoimmune disease.