Cytotoxic treatment of patients with colorectal cancer (CRC) includes most various different drugs but 5-fluorouracil (5-FU) is the cornerstone in most regimens. Leucovorin (LV) is frequently used as a part of a standard treatment regime for advanced CRC in combination with 5-FU. In numerous randomized clinical trials the addition of LV to 5-FU has been shown to significantly improve the antitumor activity and to prolong the survival of patients receiving adjuvant chemotherapy or treatment for advanced disease as to compared to treatment with 5-FU as single drug therapy.