Aim: To investigate polymorphisms that are probable indicators of response variability during cancer treatment with 5-fluorouracil (rs16430, rs2279198, rs1801159 and rs17878362). Materials and Methods: We investigated 1,038 individuals regarding allele distribution from different populations, out of which we genotyped 127 individuals from a Brazilian admixed population. Similarity analyses with parental populations were performed. Prevalence of potentially deleterious alleles was also evaluated. Results: Thirty-seven percent of the population had at least three potentially deleterious alleles and 38.6% had at least one potentially deleterious allele in homozygosis. Conclusion: Potentially deleterious alleles are present under diverse frequencies in different populations. Therefore, genotyping prior to 5-fluorouracil administration should be recommended.