OBJECTIVES: Oncotype DX® is a clinically validated assay used to guide chemotherapy decision-making for patients with early-stage breast cancer.  Patients classified as low risk by Oncotype DX® have low likelihood of benefitting from chemotherapy. By foregoing chemotherapy, patients avoid the risk of chemotherapy-related toxicities. For those patients re-classified by Oncotype DX® as high risk, the assay identifies patients who are likely to gain a large benefit from chemotherapy. The study objective was to estimate the healthcare costs of using Oncotype DX® testing in early-stage, lymph node-negative breast cancer in Ireland. METHODS: A cost-analysis estimated the health care costs (chemotherapy, administration, adverse events (AEs) and G-CSF costs) in patients whose treatment decisions are informed by Oncotype DX® testing. The perspective was that of the Irish health care system. The chemotherapy regimen was docetaxel and cyclophosphamide (4 x 21-day cycles), costing approximately €9200. Univariate sensitivity analysis was performed, together with a probabilistic sensitivity analysis (PSA) of the net reduction in chemotherapy usage from Oncotype DX® testing. In a meta-analysis of 7 published studies, there was an estimated 30% (95% CI -40%, -21%; p =0.0003) absolute reduction in chemotherapy usage after Oncotype DX® testing (ratio 0.49 [95% CI 0.41, 0.58]; p <0.00001). RESULTS: Adoption of Oncotype DX® testing resulted in approximate cost-neutrality (0.4% increase in cost) to the Irish health care system, under the above conditions. The main cost drivers were: net reduction in chemotherapy usage from Oncotype DX® testing and the rate of G-CSF usage. From the PSA, the probability of Oncotype DX® being cost-saving is approximately 47%. CONCLUSIONS: Using Oncotype DX® to inform chemotherapy decisions in early-stage breast cancer has the potential to reduce the incidence of chemotherapy-induced AEs, while being approximately cost-neutral to the Irish health care system. A cost-effectiveness analysis would be expected to result in a low incremental cost-effectiveness ratio.