Fibrous dysplasia is a benign bony abnormality that may be monostotic or polyostotic and is not included in the differential diagnosis of chest pain and dyspnea, since it is typically asymptomatic. A 36-year-old man presented with left-sided chest pain and dsypnea for three months. Chest X-ray and chest computed tomography scan revealed a large solid mass arising from the anterior parts of the left fourth, fifth, and sixth ribs and compressing the adjacent lung parenchyma. The tumor was completely removed surgically and histopathologic examination was consistent with fibrous dysplasia without malignant transformation. This report demonstrates that polyostotic dysplasia of the ribs may cause chest pain and dyspnea and present radiographically as a large mass.