Background: The fluoropyrimidines (FP), 5-Fluorouracil (5-FU) and capecitabine are a mainstay of colorectal cancer (CRC) treatment. Dihydropyrimidine dehydrogenase (DPYD), an enzyme encoded by the DPYD gene, is the initial and rate-limiting step in pyrimidine catabolism, deactivating over 80% of 5-FU. Approximately 5% of the population are deficient in DPYD and can develop severe or fatal FP toxicities. Currently, few national guidelines recommend routine prospective DPYD testing. In July 2019, we commenced a 6 month prospective pilot, testing DPYD status of all CRC patients undergoing first FP treatment in a large regional cancer centre. Methods: All CRC patients eligible for first exposure to FP are tested using a rapid molecular assay screening for five SNPs (detects 70% of DPYD mutations) and we will present data on prevalence of each. We will use electronic chemotherapy prescribing records (July 19-Jan 20) to collect information on dose modifications and toxicities. Once the pilot is completed we will perform a cost-effectiveness analysis. Results: Data from the first 3 months of this pilot have been reviewed and 201 patients have been tested with 15 heterozygotes identified, of which 2 had more than one mutation. No homozygotes were found. All heterozygote patients are started with a dose reduction (or have alternative therapy). One patient treated at 50% dose was hospitalised with several grade 3 toxicities despite dose reduction. Two patients have had subsequent dose escalation (by 25%). Nine patients have received one dose reduced cycle without complication. Three patients are due to start dose-reduced treatment. Conclusions: Routine prospective testing of DPYD status in a large regional cancer centre is feasible and with a sufficiently swift result turnaround to permit up-front dose modification. Detailed toxicity analysis and cost-effectiveness data will be presented.