Introduction: Breast cancer is formed from breast cells, either lobule or duct epithelium, which undergoes uncontrolled growth and development. The therapy of breast cancer to reduce mortality and morbidity can be done by way of surgery, chemotherapy, and radiotherapy. Doxorubicin chemotherapy is a therapeutic option to kill cells directly or by stopping the cell division based on the tumor pathogenesis. Method: This research is a paired numerical comparative analytical study using a retrospective cohort approach. This study was conducted on 56 breast cancer patients to see the differences in myelosuppression before and after doxorubicin chemotherapy at H. Adam Malik General Hospital Medan from January-December 2019. The data were obtained through recording the routine blood laboratory results listed in the patient's medical record and were analyzed with dependent t-test by using SPSS. The difference is significant if p <0.05. Result: The results showed that the average age of the research subjects was 40.79 years, 28 patients (50%) graduated from high school, 38 patients (67.9%) did not work and 53 patients (94.6%) had marital status. In a significant myelosuppression difference, anemia occured with chemotherapy Hb levels of 8.48 ± 1.09, the Hb difference in Hb prior to and after doxorubicin chemotherapy obtained a mean of 1.28 with p value <0.001. Conclusion: Based on the demographic characteristics of patients with breast cancer, it was found that the mean age of patients with breast cancer in women was 46.79 years; most of the subject’s education level was highschool, unemployed and married status. There was a very significant difference between the subject's Hb level and breast cancer before and after doxorubicin chemotherapy. Keywords: myelosuppression, doxorubicin, breast cancer.