Herein we describe the antiproliferative effects of two natural dibenzo[b,f]oxepines, pacharin (PAC) and bauhiniastatin-1 (BAU), isolated from Bauhinia acuruana on breast cancer cell line and the mode of action underlying the cytotoxicity. Both compounds were cytotoxic in a panel of six tumor lines analyzed by MTT assay and IC50 values ranged from 7.8 to 45.1 μM, including upon human breast adenocarcinoma (MCF-7) cells. In contrast, none of the compounds was cytotoxic on normal human peripheral blood mononuclear cells (IC50>100 μM). Then, MCF-7 cells treated with PAC and BAU 20 µM during 24h presented a reduction in cell volume and intensification of chromatin condensation, DNA fragmentation and apoptotic cells, whose findings became more evident after 48h of exposure. Anti-apoptotic B-cell lymphoma-2 (BCL-2) family members, such as Myeloid cell leukemia-1 (MCL-1) and B-cell lymphoma-extra large (Bcl-xL) are important targets in cancer cell, since their overexpression confer resistance to cancer treatments. A significant reduction of the MCL-1 protein levels in MCF-7 cells after 24h of treatment with PAC and BAU at 20 μM was observed, while the Bcl-xL protein content was reduced in BAU-treated cells at 40 μM only. The cytotoxic effects of PAC and BAU are likely linked to the MCL-1 inhibition, which leads to the apoptosis of breast adenocarcinoma cells.