Studies have demonstrated that body surface area-based dosing of chemotherapy drugs leads to significant individual exposure variability, with a substantial risk of under- or overdosing. The present study was initiated to validate the use of therapeutic drug management (TDM) to personalize 5-fluorouracil (5-FU) dosing in patients with metastatic colorectal cancer treated in routine clinical practice.A total of 75 patients with metastatic colorectal cancer from 8 German medical centers received ≤ 6 administrations of infusional 5-FU according to the AIO (folinate, 5-FU; n = 16), FOLFOX6 (leucovorin calcium [folinic acid], 5-FU, and oxaliplatin; n = 26), or FUFOX (oxaliplatin plus 5-FU/folinic acid; n = 33) regimen. Initial infusional 5-FU dosing for all patients was determined by the BSA. Individual 5-FU exposure (area under the curve [AUC]) was measured using an immunoassay of a blood sample taken during each infusion. To achieve a target AUC of 20 to 30 mg × h/L, subsequent infusional 5-FU doses were adjusted according to the previous cycle's 5-FU AUC. The primary objective was to confirm that TDM of infusional 5-FU resulted in an increased proportion of patients in the target AUC range at the fourth versus the first administration. The secondary objective was to determine whether 5-FU TDM reduced the treatment-related toxicities compared with the historical data.The average 5-FU AUC at the first administration was 18 ± 6 mg × h/L, with 64%, 33%, and 3% of the patients below, within, or above the target AUC range, respectively. By the fourth administration, the average 5-FU AUC was 25 ± 7 mg × h/L (P < .001), with 54% of patients within the target 5-FU AUC range (P = .0294). The incidence of 5-FU-related grade 3 and 4 diarrhea (4.6%), nausea (3.4%), fatigue (0.0%), and mucositis (0.2%) was reduced compared with the historical data, despite 55% of the patients receiving increased doses.Personalization of 5-FU dosing using TDM in routine clinical practice resulted in significantly improved 5-FU exposure and suggested a lower incidence of 5-FU-related toxicities.