We report a case of a breast cancer patient with bone marrow carcinomatosis and disseminated intravascular coagulation who was treated with chemotherapy and a CDK4/6 inhibitor. The patients, a 68-year-old woman, presented to our hospital with anorexia and was found to have multiple liver metastases of breast cancer. Furthermore, she had anemia and thrombocytopenia, and a bone marrow biopsy showed bone metastasis of the breast cancer. Therefore, a diagnosis of bone marrow carcinomatosis and disseminated intravascular coagulation was made. Treatment was started with chemotherapy(epirubicin and cyclophosphamide)and subsequently changed to an aromatase inhibitor(letrozole)and a CDK4/4 inhibitor(abemaciclib) and was maintained without exacerbation of the patient's condition.