A single arm study in metastatic colorectal cancer patients treated with pharmacokinetically (PK) dose adjusted weekly or biweekly 5-fluorouracil (5-FU) regimes.

To determine whether pharmacokinetically-guided dose adjustment of 5-FU provides a stable intrapatient dose level of 20-30 mg.h /l. The primary analysis will be the comparison of the proportion of patients with AUC within 20 to 30 mg.h/L after the first 5-FU application versus the fourth application.