Adjuvant chemotherapy has been established since 1990 as standard treatment for patients with colon cancer stage III (Dukes' C). Chemotherapeutic schemes combining 5-fluorouracil with levamisole or leucovorin have shown significant advantage over surgery alone. Adjuvant trials are being currently implemented to investigate some relevant questions, such as which is the optimal duration of chemotherapy, as well as the possible advantage of levamisol versus leucovorin schedules, and of high-dose versus low-dose leucovorin. While these trials are ongoing, a retrospective evaluation of the toxicity associated with the different chemotherapeutic schemes might be of help when choosing the most appropriate regimen for individual patients not involved in clinical trials. A total of 519 patients subjected to three different schedules of adjuvant chemotherapy between 1993 and 1996, were evaluated for toxicity according to the NCI-CTC criteria. Chemotherapeutic regimens were: 5-fluorouracil plus levamisole (5-Fu+Lev; Moertel schedule), 5-fluorouracil plus low-dose leucovorin (5-Fu+LVLD; NCCTG schedule) and 5-fluorouracil plus high-dose leucovorin (5-Fu+LVHD; IMPACT-modified schedule). 5-Fu+LVLD is significantly more toxic than the other two regimens in terms of neutropenia, mucositis and diarrhea. delay in chemotherapy and dose reduction of 5-fluorouracil were also more frequent in the 5-Fu+LVLD group. However, the percentage of prematurely discontinued treatments was significantly higher in the 5-Fu+Lev group. Information on toxicity of adjuvant chemotherapy for colon cancer may help medical oncologists to choose the most appropriate regimen for individual patients not involved in clinical trials.