We report a case of pyoderma gangrenosum (PG) mimicking a severe infectious skin disease in a woman with metastatic breast cancer. PG started at the site of an intramuscular injection administered a few days previously, and it subsequently extended. The skin disease was cured by high-dose corticosteroid therapy and clofazimine, but it marked a turn for the worst in the course of the breast cancer which became rapidly fatal.