Description: Background: Breast-cancer incidence and mortality have been increasing in Japan. Japanese-specific clinical validity and utility data for the 21-gene assay (Oncotype DX(R) Breast Cancer Assay; Genomic Health, Inc., Redwood City, USA) are now available. The objective of this study was to evaluate the cost-effectiveness of the 21-gene assay for the guidance of adjuvant chemotherapy decisions in estrogen-receptor-positive, lymph-node-negative, early-stage breast cancer patients, from the Japanese societal perspective. Methods: The recurrence risk group distribution by the 21-gene assay result and the assay's influence on adjuvant chemotherapy recommendations were obtained from a study of 104 patients. A state-transition cohort (Markov) model tracked time from surgery until distant recurrence and from distant recurrence to death. Adjuvant chemotherapy benefit by 21-gene assay risk group was based on published clinical validation studies. Direct and indirect medical costs were obtained from the referral centers. Utilities associated with progression and chemotherapy-related adverse events were extracted from literature. Sensitivity analyses assessed the key drivers and robustness of the primary outcomes. Results: The 21-gene assay identified 48% of patients as low-risk, 36% as intermediate-risk, and 16% as high-risk. Total acute chemotherapy-related costs decreased by [yen sign]154,066 due to less adjuvant chemotherapy usage. In the high-risk group, adjuvant chemotherapy use increased 18%, leading to survival benefits. Chemotherapy use overall decreased by 19%. Monitoring costs increased by [yen sign]3,744 but recurrence costs declined by [yen sign]46,113 per patient. Use of the 21-gene assay increased quality-adjusted-life-years (QALYs) by 0.241 per patient on average; the net cost per QALY gained was [yen sign]636,752 ($6,368). Conclusions: The 21-gene assay for women with estrogen-receptor-positive, lymph-node-negative, early-stage breast cancer is projected to be cost-effective in Japan.