Active specific immunotherapy, using vaccines with autologous tumour cells and BCG, significantly reduces the rate of tumour recurrence in stage II colon cancer patients, while no clinical benefit has yet been observed in stage III patients. Adjuvant treatment with 5-Fluorouracil/Leucovorin is now considered standard therapy for stage III colon carcinoma and results in an absolute survival benefit of approximately 10%. Yet, the 5-year overall survival rate of stage III colon cancer patients is only 40 -50%. Combining chemotherapy and immunotherapy might improve prognosis for stage III patients, especially when considering that active specific immunotherapy and chemotherapy have shown synergistic effects in pre-clinical tumour models. We performed a phase II study with 56 patients, using the combination of active specific immunotherapy and chemotherapy as an adjuvant therapy in stage III colon cancer patients to assess the influence of 5-Fluorouracil/Leucovorin on anti-tumour immunity induced by autologous tumour cell vaccinations. Anti-tumour immunity was measured before and after chemotherapy by means of delayed type hypersensitivity reactions, taken 48 h after the third and the fourth vaccination. We also investigated the toxicity of this combined immuno-chemotherapy treatment. Delayed type hypersensitivity reactions before chemotherapy had a median size of 20.3 mm, while after chemotherapy delayed type hypersensitivity size was 18.4 mm (P=0.01), indicating that chemotherapy hardly affected anti-tumour immunity. The severity of ulcers at the BCG vaccination sites was comparable to previous studies. In 30% of the patients grade III or grade IV chemotherapy related toxicity was seen; this is comparable to what is normally observed after adjuvant chemotherapy alone. This study shows that the active specific immunotherapy-induced anti-tumour immune response is only minimally impaired by consecutive chemotherapy and that the combined treatment of stage III colon cancer patients with active specific immunotherapy and 5-Fluorouracil/Leucovorin does not cause unexpected toxicity. Colon cancer is one of the most prevalent malignancies in the industrialised world, with approximately 240 000 new cases diagnosed each year in the USA and Western Europe. When colon cancer is detected early, in stages I or II (Dukes A or B), the 5 year survival rate with surgery alone is 60 -90%, depending on the depth of invasion of the tumour into the bowel wall. When the cancer has spread to the regional lymph nodes, or stage III (Dukes C), the 5 year survival rate is 25 -60%. Important prognostic factors in stage III colon carcinoma include the number of positive nodes, extra-nodal growth, invasion of adjacent organs, grade of histologic differentiation and post-operative CEA level. When distant metastases are diagnosed, stage IV (Dukes D), the 5 year survival rate is less than 5% (Cohen et al, 1997).Currently most patients with stage III colon carcinoma receive an adjuvant treatment with 5-Fluorouracil (5-FU) and Leucovorin. Several lar...