Pyomyositis is a rare complication of chemotherapy. A 47-year-old woman with metastatic breast cancer, in whom pyomyositis developed after chemotherapy, is described. It was difficult to differentiate between pyomyositis and deep venous thrombosis early in her admission. Pyomyositis should be considered part of the differential diagnosis of deep venous thrombosis. This infection, after chemotherapy, usually is considered to be caused by neutropenia or immunodeficiency secondary to the cancer, or both. It is postulated that subclinical myopathy, secondary to the malignancy or drugs used in treating the malignancy, or both, may also predispose to pyomyositis.