In 10-30% of colorectal cancer (CRC) patients, toxic reactions occur after fluoropyrimidine-based chemotherapy. A dihydropyridine dehydrogenase ( gene variant, c.1905 + 1G>A, leads to intolerance to fluoropyrimidines. Due to the low frequency of this variant in many populations, the prevalence of fluoropyrimidine-induced hematologic side effects in CRC patients with the c.1905 + 1G>A variant is unclear. In this study, we investigated the prevalence of the c.1905 + 1 variants in a Turkish CRC cohort and the potential effects of these variants on fluoropyrimidine-induced hematologic side effects. The c.1905 + 1 variant was genotyped using polymerase chain reaction-restriction fragment length polymorphism analysis and confirmed by Sanger sequencing in peripheral blood samples of 100 CRC patients who received fluoropyrimidine-based chemotherapy and 60 healthy volunteers. The association of c.1905 + 1 variants with susceptibility to hematologic side effects was evaluated. The c.1905 + 1G>A variant was more common in the CRC group than in the healthy control group ( = 0.001). The presence of the c.1905 + 1G>A variant was associated with thrombocytopenia ( = 0.039) and anemia ( = 0.035). CRC patients with fluoropyrimidine-induced anemia had shorter disease-free survival than CRC patients without fluoropyrimidine-induced anemia ( = 0.0009). Before administering fluoropyrimidine-based chemotherapy, genetic screening for the c.1905 + 1G>A variant should be performed with the aim of preventing anemia and anemia-induced complications in CRC patients.