The 21-gene reverse transcriptase-polymerase chain reaction assay with a patented algorithm is validated as a good predictor of prognosis and potential benefit from adjuvant chemotherapy for lymph-node-negative, estrogen-receptor-positive, early-stage breast cancer, while its high cost raises concern about how to finance it. Cost-effectiveness analysis comparing prevalent National Comprehensive Cancer Network (NCCN) guideline/St Gallen recommendation-guided treatment with the assay-guided treatment is carried out with budget impact estimation in the context of Japan's health care system. Incremental cost-effectiveness ratios are estimated as 2,997,495 yen/QALY (26,065 US$/QALY) in the comparison between NCCN guided-treatment vs. the assay-guided treatment, and as 1,239,055 yen/QALY (10,774 US$/QALY) in the comparison between St Gallen guided-treatment vs. the assay-guided treatment. Budget impact is estimated as yen2,638 million (US$23 million) to yen3,225 million (US$28 million) per year. The routine use of the assay is indicated as cost-effective. And the budget impact could be judged as within fundable level.