In metastatic colorectal and other locally advanced gastrointestinal cancers, the mechanisms of tumor growth and/or immune escape by residual cancer cells after curative resection often provoke tumor recurrence. Current adjuvant therapy is based on pharmacological administration up to 6-8 months after surgery. We hypothesized that the long-term, cytostatic action from repeated post-adjuvant administration of 5-fluorouracil (FU)-leucovorin (LV) cycles, as a result of the downregulation of the above-mentioned cellular mechanisms, could halt tumor progression. An active prospective cohort, including 19 patients (study group) at high risk of relapse, was considered. All patients received repeated post-adjuvant administration of 5-FU-LV cycles for up to 52-60 months following curative surgery (total cumulative dose of about 90 g and mean follow-up of 70.6 ± 49.7 months). The 5-year disease-free interval (DFS) and overall survival (OS) were 80.4 ± 10.2% and 87.1 ± 8.6%, respectively, which is very different from the recent literature that has reported 5-year DFS and OS values of 31.8% and 40.1%, respectively. These findings suggest that this new pharmacological approach based on the long-term maintenance of a cytostatic effect with 5-FU-LV can produce a relevant improvement in the outcome of this population.