Background: Dihydropyrimidine dehydrogenase (DPD) is a key enzyme in the metabolism of fluorouracil. Deleterious polymorphisms in gene-encoding DPD (DPYD) results in a DPD deficiency that causes life-threatening toxicities when the standard dose of fluorouracil is used. DPYD*2A (IVS14+1G > A) is the most common single-nucleotide polymorphism (SNP) associated with critical DPD deficiency. At present, most of the evidence supports screening for at least 3 SNPs (DPYD*2A, c.2846 A > T, c.1679T > G). The aim of this study is to confirm that the detection of additional polymorphisms of DPYD could enhance prevention of fluoropyrimidine toxicity. Methods: In 2011, we began to screen DPYD*2A in patients candidate for fluoropyrimidine based-chemotherapy. As the first step of the evaluation, we selected all cases of DPYD*2A wild type, from 2011 to 2012, who developed CTC-NCI-V.3 toxicity ≥ G3. In these patients, we researched the other 3 SNPs (c.2846 A > T, c.1679T > G, c.2194C > A). Mutational status was analyzed with real Time PCR. Results: From 2011 to 2016 we pre-emptively screened DPD deficiency in 1,863 patients and 32 subjects (1.6%), with results mutated for DPYD*2A. As the first step of the evaluation, 548 subjects were assessed from 2011 to 2012. We found 7 patients who were carriers of the DPYD*2A mutation (1.27%). Of the 541 wild type cases, 114 presented toxicities ≥ G3. In this subgroup, 22 patients (19%) proved to be mutated for the other SNPs of DPYP, as reported in the table below. Conclusions: Preliminary data show that in 22 (19%) of 114 patients who presented severe toxicity which was not correlated with DPYD*2A, we found other polymorphisms of gene encoding DPD. Out of the 3 SNPs evaluated, c.2194 C > A proved to be the most frequent, although it is the polymorphism that is least known and least studied. Such results suggest that the evaluation of additional polymorphisms could enhance the prevention of fluoropyrimidine toxicity. The results are expected to be clarified further in the second step, which is ongoing.