The dihydrouracil (DHU):uracil (U) plasma ratio is a promising marker for identification of dihydropyrimidine dehydrogenase (DPD)-deficient patients. The objective of this study was to determine the effect of liver resection on the DHU:U plasma ratio in patients with colorectal liver metastases (CRLM).An observational study was performed in which DHU:U plasma ratios in patients with CRLM were analyzed prior to and 1 day after liver resection. In addition, the DHU:U plasma ratio was quantified in six additional patients 4-8 weeks after liver resection to explore long-term effects on the DHU:U plasma ratio. Quantification of U and DHU plasma levels was performed using a validated ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) assay.The median (range) DHU:U plasma ratio in 15 patients prior to liver resection was 10.7 (2.6-14.4) and was significantly reduced to 5.5 (< quantification limit (LLOQ-10.5) 1 day after resection (p = 0.0026). This reduction was caused by a decrease in DHU plasma levels from 112.0 (79.8-153) ng/mL to 41.2 (< LLOQ-160) ng/mL 1 day after resection (p = 0.0004). Recovery of the DHU:U plasma ratio occurred 4-8 weeks after liver resection, which was shown by a median (range) DHU:U plasma ratio in six patients of 9.1 (6.9-14.5).Liver resection leads to very low DHU:U plasma ratios 1 day after liver resection, which is possibly caused by a reduction in DPD activity. Quantification of the DHU:U plasma ratios directly after liver resection could lead to false-positive identification of DPD deficiency and is therefore not advised.