In metastatic colorectal cancer the oral 5-fluorouracil (5FU) prodrug capecitabine is used with increasing frequency as an alternative to i.v. 5FU administration. The rate of conversion of capecitabine into 5'deoxy-5-fluorouridine has been related to tumor response, and 5FU catabolites have been associated with 5FU-related systemic toxicity. Here we demonstrate for the first time that capecitabine, its metabolites 5'deoxy-5-fluorocytidine and 5'deoxy-5-fluorouridine, and its catabolites 5-fluoro-ureido-propionic acid, alpha-fluoro-beta-alanine, and alpha-fluoro-beta-alanine-bile acid conjugate can be monitored in vivo by (19)fluorine magnetic resonance spectroscopy ((19)F MRS) in the liver of patients with metastatic colorectal cancer. Moreover, we demonstrate an improved signal-to-noise ratio and spectral resolution of the (19)F MRS spectra when measurements are performed at 3 T field strength as compared with measurements at the common clinical field strength of 1.5 T. We conclude that assessment of capecitabine metabolism in patients by (19)F MRS is a promising noninvasive tool for the prediction of its efficacy and toxicity, especially at the now currently available clinical field strength of 3 T.