Fever of unknown origin (FUO) characterizes febrile disorders that are accompanied by prolonged fevers of 101 degrees F or greater for 3 weeks or more that remain undiagnosed after comprehensive inpatient and outpatient diagnostic testing. At the present time, malignancies are the most common cause of FUOs. Among malignant FUOs, lymphomas are the most common. We present the case of a non-Asian young adult man who presented with FUO. He had no peripheral adenopathy or splenomegaly but was found to have anterior/superior mediastinal adenopathy and right paratracheal adenopathy. His diagnostic workup was negative for rheumatic/inflammatory and infectious diseases. Laboratory test results were unremarkable except for a highly elevated erythrocyte sedimentation rate and highly elevated serum ferritin level. Otherwise unexplained highly elevated serum ferritin levels in patients with FUOs suggest rheumatic and inflammatory disorders, for example, systemic lupus erythematosus flare or malignancy. The findings of mediastinal adenopathy combined with a highly elevated ESR and highly elevated serum ferritin levels indicate lymphoma as the most likely diagnosis. He also had polyclonal gammopathy on serum protein electrophoresis (SPEP). In a patient with FUO, negative blood cultures, and a heart murmur, polyclonal gammopathy on SPEP suggests atrial myxoma. Lymphomas are often associated with elevated alpha(1)/alpha(2) globulins on SPEP. Lymph node biopsy of the mediastinal nodes was negative for lymphoma but did not show characteristic emperiopolesis, pathognomonic of Rosai-Dorfman disease, a benign lymphoproliferative disorder. Rosai-Dorfman disease usually presents with massive bilateral cervical adenopathy but may present with lymph node involvement in other sites, as in this case. In patients with lymphadenopathy and a negative FUO workup, clinicians should consider the possibility of Rosai-Dorfman disease, particularly if accompanied by an otherwise unexplained highly elevated serum ferritin levels and polyclonal gammopathy on SPEP.