Background: The objectives of this study were to estimate and cross-nationally compare the medical costs shared by payers and patients and the distributions of medical costs by cost category. Material and Methods: We estimated the medical costs covered from definitive diagnosis to completion of treatments of early-stage breast cancer and follow-up, assuming almost identical medical care provided in Japan, the UK, and Germany. The analysis was performed from the payer's perspective. Medical costs were calculated by multiplying the unit costs by the number of units consumed, based on assumption case scenarios. The medical costs incurred by payers or patients were estimated according to the cost-sharing and the cost-bearing systems in each country. Results: The total medical costs in Japan were much lower than those in the UK and Germany, and these differences were mainly caused by the low costs of surgery and radiotherapy in Japan. For the base-case scenario, the co-payment in Japan (€ 3,486) was found to be 6.4-fold higher than that in Germany (€ 548). The payers in the European countries paid 2.9-fold more than those in Japan (€ ∼25,000 vs. € 8,627). Conclusion: Our results will be useful for policy makers in considering how to share medical costs and how to allocate limited resources.