During the last decade, the National Surgical Adjuvant Breast and Bowel Project (NSABP) has completed six adjuvant chemotherapy trials comparing different adjuvant therapy regimens or adjuvant therapy versus surgery alone. A seventh trial is ongoing. These trials have contributed to defining the role of adjuvant therapy in colon cancer. Patients eligible for inclusion in NSABP trials had been diagnosed as having stage II or III colon cancer with no evidence of gross residual or metastatic disease. The follow-up strategies were similar in the reported trials with follow-up every 3 months for the first 2 years, then every 6 months for the next 3 to 5 years, and annually thereafter. The NSABP C-01 protocol was a three-arm trial comparing an adjuvant semustine/vincristine/5-fluorouracil (5-FU) regimen (MOF) to a Bacille Calmette-Guerin treatment, and to surgery alone. The C-02 protocol investigated whether portal vein infusion of 5-FU improved survival outcome compared with surgery alone. Protocol C-03 compared a semustine/vincristine/5-FU regimen to a 5-FU plus leucovorin (LV) (5-FU/LV) regimen. The NSABP C-04 protocol was a three-arm trial comparing 5-FU/LV, 5-FU plus levamisole, and 5-FU/LV plus levamisole. The NSABP C-05 trial compared 5-FU/LV to 5-FU/LV plus alpha-interferon. Results of NSABP C-01, C-02, C-03, C-04, and C-05 trials are summarized in this report. Patient accrual has completed in the NSABP C-06 trial comparing 5-FU/LV with oral tegafur and plus uracil leucovorin. The NSABP is currently conducting another trial (C-07) comparing 5-FU/LV with 5-FU/LV plus oxaliplatin. The role of adjuvant chemotherapy in stage II colon cancer is also discussed in this report. A recent pooled analysis of studies C-01, C-02, C-03, and C-04 has indicated that the relative treatment benefit in stage II disease is at least equal to the benefit in stage III colon cancers, and concluded that adjuvant chemotherapy also should be considered as the standard of care for stage II colon cancer patients.