We report two patients admitted to our hospital suspected to suffer from cancer in the lung or mediastinum, respectively. Both patients had a diagnosis of thoracic actinomycosis. A 76 year old man revealed pulmonary and endobronchial actinomycosis associated with broncholithiasis. Diagnosis was achieved by bronchoscopy. Therapy with ampicillin/sulbactam was successful. A 36 year old patient presented with bilateral pleural effusions, extended pericardial, mediastinal and pulmonary actinomycosis with pericarditis constrictiva and superior vena cava syndrome. Diagnosis was finally made by cardiac surgery with therapeutic pericardectomy. Prolonged therapy with ampicillin/sulbactam was administered with satisfactory result. Here we discuss the importance to include actinomycosis in the differential diagnosis of pulmonary affections and mediastinal masses in order to avoid diagnostic errors and to limit invasive procedures to the necessary amount. We illustrate the need of an individualized treatment approach.