Capecitabine, 5-fluorouracil and tegafur form the group called the fluoropyrimidines, which is one of the most frequently prescribed group of anti-cancer drugs for the treatment of (metastatic) colorectal, gastric and breast cancer. The primary enzyme responsible for the inactivation of the fluoropyrimidines is dihydropyrimidine dehydrogenase (DPD). Consequently, patients with an inborn partial DPD deficiency, induced, for example by the polymorphism DPYD*2A, are highly prone to severe, potentially lethal toxicity following a standard dose of fluoropyrimidines. In this article, based on three representative case reports and our prospective study in patients with cancer, we demonstrate the clinical value of prospective screening for DPD deficiency in patients being treated with fluoropyrimidine-based anti-cancer therapy. The results show that upfront genotyping for DPYD*2A followed by a fluoropyrimidine dose reduction of 50% (on average) in patients heterozygous polymorphic for DPYD*2A, significantly reduces the incidence of severe to potentially lethal toxicity compared to historical control patients given full-dose therapy.