Breast cancer is the most commonly diagnosed malignancy in women, but little is known about therapeutic outcomes in patients with both breast cancer and HIV. We performed a retrospective cohort study of women with or without HIV undergoing treatment for breast cancer from 1996 to 2011. Cases with HIV were 1:2 matched to non-HIV controls based on age, sex, race, and date of cancer diagnosis. Dose reduction and/or delay during chemotherapy, overall survival, and development of metastatic disease were studied outcomes. 156 (52 HIV, 104 non-HIV) subjects were analyzed. The majority of breast cancers in both groups were clinical stages 0, I, II, and III (73%). HIV infection preceded cancer diagnosis by a median of 13 years. Median CD4 count at time of cancer diagnosis was 417 cells/mcL. Approximately 87% (45/52) were on HAART, mostly protease inhibitor-based (57%) therapy. HIV-infected women needed more dose reductions and/or delays to chemotherapy due to toxicity (56% vs. 30%; p=0.03). Stage at diagnosis, triple negative receptor status, and dose reduction and/or delay were predictors of metastatic disease and death. HIV-infected women experienced more adverse events during breast cancer treatment, and a potential causative factor could be drug-drug interactions between HAART and chemotherapy.