The rational of leucovorin modulation of 5-fluorouracil and the clinical results in colorectal cancer are reviewed with special emphasis on the monthly schedule of low dose leucovorin and 5FU bolus for 5 consecutive days (NCCTG-Mayo Clinic regimen) and the bimonthly schedule of high-dose leucovorin and 5FU bolus plus continuous infusion for two consecutive days (LV5FU2) which is now considered as a new standard.