A phase II prospective trial was carried out to study the concept of 5-fluorouracil (5-FU) dose-intensity in patients with advanced colorectal cancer. Forty patients were treated with 5-FU plus leucovorin (LV), with individually increasing doses of 5-FU. A 5-FU pharmacokinetic follow up was performed and a relationship was sought between its metabolism and its response to treatment, and between 5-FU's toxicity and patient survival.