Many types of infections associated with colorectal cancer have been reported. Here, we describe a rare case of thoracic empyema that was observed during immunotherapy for recurrent colon cancer. Culture of the pleural fluid yielded Streptococcus bovis, which is known to be associated with gastrointestinal lesions, especially colorectal malignancies. The possible correlation between these two clinical entities-empyema and colon cancer-is discussed.