A severe, post-mediansternotomy, mediastinitis was treated with pectoral muscle flap method. About 10 days after CABG this patient had purulent discharge, high fever, and unstable sternum. Under a diagnosis of mediastinitis, wound irrigation and systemic administration with antibiotics began, but these managements were ineffective. Therefore, we closed the chest primarily by the pectoral muscle flap method. Successful primary closure of the chest was accomplished.