THERATOPE (Biomira Inc., Edmonton, AB, Canada) STn-KLH cancer vaccine induces strong antibody titers against both the synthetic STn epitope and against a natural mucin, OSM, which expresses STn-like epitopes. In prospective, randomized studies in patients with metastatic breast cancer treated at two cancer centers, the effect of different low-dose, immunomodulatory cyclophosphamide (cyclo) pretreatments on the response to THERATOPE STn-KLH was compared. Patients were randomized to receive either intravenous cyclo 300 mg/m2 on day -3, or oral cyclo 50 mg daily from days -14 to -3 inclusive, or no cyclo, before THERATOPE treatments. The anti-STn and anti-OSM antibody titers were higher in the patients who received cyclo intravenously before THERATOPE. Patients treated with cyclo intravenously and THERATOPE STn-KLH cancer vaccine lived significantly longer (projected median survival of 19.7 months versus actual median survival of 12.6 months, p = 0.0176) than those treated with the same STn vaccine with oral or no cyclo. Although it is not clear how the anti-STn antibody response modifies tumor biology, we noted that patients in the intravenously administered cyclo group had a lower percentage of patients showing progressive disease at 9 weeks, and that there was an inverse correlation between serum anti-STN antibody titer and growth of measurable tumors. There was no correlation between tumor growth and anti-KLH antibody titers. These data are consistent with a therapeutic effect of THERATOPE STn-KLH cancer vaccine and support development of a phase III study to explore this further.