We report the magnetic resonance imaging (MRI) and positron emission tomography/computed tomography (PET/CT) findings of a patient who presented with lower thoracic pain, nausea, and vomiting and had a CT scan of the chest in which a well-defined homogeneous mass was identified. Further evaluation with MRI and PET/CT showed a nonenhancing homogeneous paraspinal mass in the left lung base involving the left hemidiaphragm. The mass showed lack of enhancement on MRI and was photopenic on PET/CT. Video-assisted thoracoscopic surgery and resection of the mass was performed and showed a mesenchymal cystic hamartoma invading the left hemidiaphragm. Such a growing hamartoma with pleural and diaphragmatic invasion is very atypical and to our knowledge, has not been previously reported.