Active specific immunotherapy (ASI) is a promising approach to treating cancer. Numerous studies in the laboratory have demonstrated that various cancer vaccines can stimulate antibody and cell mediated immune responses against tumour-associated antigens [1-9]. Yet few studies have demonstrated convincing clinical responses. Sialyl-Tn (STn) is a carbohydrate associated with the MUC1 mucin on a number of human cancer cells and is associated with more aggressive disease. Consequently, STn is an ideal candidate for ASI therapy. Theratope vaccine is a cancer vaccine that was designed by Biomira, Inc. (Edmonton, Alberta, Canada) by incorporating a synthetic STn antigen that emulates the carbohydrate seen on human tumours. The clinical trials conducted to date with Theratope vaccine are outlined in this report. Overall, Theratope vaccine has been well-tolerated with minimal toxicity. The most common side effects have been in duration and erythema at the site of injections. Both in a non-transplant setting following low dose iv. cyclophosphamide and high dose autologous transplant setting, there has been a trend toward Theratope vaccine decreasing the risk for relapse, prolonging the time to relapse and thus impacting on overall survival. The definitive Phase III trial comparing the outcome of patients with metastatic breast cancer receiving vaccinations with Theratope vaccine versus vaccination with the nonspecific immune stimulants Keyhole Limpet Hemocyanin (KLH) and Detox -B stable emulsion (Detox-B) (now called Enhanzyn Immunostimulant) was closed to enrolment on March 30, 2001. Over 1000 women with distant metastatic breast cancer were enrolled into the program.