OBJECTIVES: To describe costs associated to the treatment of different stages of breast cancer patients at the Social Security Mexican Institute (IMSS) from the health care payer's perspective. METHODS: A cost study was elaborated. Resource use and cost data were obtained from hospital (second and tertiary levels) records of 313 of treated patients during July 2008 to February 2009 using the following inclusion criteria: women older than 16 years with breast cancer histological diagnosis who accepted to be included in the protocol through informed consent. Although, patients excluded were those who showed a second malignant neoplasm or incomplete information. We calculate mean, median, 95% confidence interval (95% CI) for each clinical stage and statistical differences were estimated through ANOVA tests, p value <0.05 was considered significant to show differences. RESULTS: The median total cost per patient was found in US $6135.38 (95% CI, US $4216.19 - US $9737.19); the median cost per chemotherapy cycle was US $615.48 (95% CI, US $425.98 - US $1456.63); all chemotherapy treatment resulted in US $2702.03, (95% CI, US $1456.36 - US $5503.49) and median costs per patient with radiotherapy resulted in US $1260.78 (95% CI, US $421.34 - US $1260.78). The mean cost per patient in each clinical stage with chemotherapy was: I: US $1830.80 (95% CI, US $686.21 - $2975.39); II: US $5143.41 (95% CI, US $3570.19 - $6716.62); III: US $4079.77 (95% CI, US $2739.86 - $5419.68); IV: US $4907.21 (95% CI, US $672.11 - $9142.31) and the non classified patients: US $5250.66 (95% CI, US $3360.94 - $7140.40); p=0.401. CONCLUSIONS: The results showed that at the IMSS, it is more expensive the treatment of breast cancer patients in clinical stage II; however, the less expensive treatments resulted for patients in clinical stage I. In addition, the treatment of non classified patients were the second most expensive according to our results.