Three cases of primary Rosai-Dorfman disease of the thymus and lung are presented. The patients are 3 men between the ages of 42 and 47 years who presented with non-specific symptoms including cough, chest pain, and shortness of breath. Clinically, the patients did not have any other pertinent clinical history. Diagnostic imaging revealed in one patient a cystic anterior mediastinal mass, while in two other patients the imaging was that of an intrapulmonary mass, one in the right upper lobe and the other in the left lower lobe. The three patients undergo surgical resection of the mass. In the cases in which the tumor mass was in the lung, both patients had a lobectomy while in the patient with anterior mediastinal mass, surgical resection via thoracotomy was performed. The intrapulmonary tumors were described as soft and yellowish measuring 2.5 and 3.0 cm in greatest diameter, while the mediastinal mass was described as cystic measuring 4.0 cm in diameter. Histologically, all tumors show similar features in terms of a proliferation of large histiocytes admixed with an inflammatory component composed predominantly of plasma cells. Immunohistochemical stains show positive staining for CD68 and S-100 protein, while negative for keratin, CD1a, and langerin. The cases herein presented highlight the ubiquitous distribution of Rosai-Dorfman disease and the importance of keeping this entity in the differential diagnosis of histiocytic proliferation in the thymus or lung.