A 75-year-old female with a history of stomach cancer and depression was referred to our hospital for left cervical lymphadenopathy. The biopsy of her left cervical lymph node revealed noncaseating granulomas with multinucleated giant cells. The positron emission tomography/computed tomography (PET/CT) indicated general lymphadenopathy (left supraclavicular left axillary, hepatic lymph nodes), except for the hilar lymph node. Both histology by transbronchial lung biopsy (TBLB) and analysis of broncho alveolar lavage fluid (BALF) were indicative of sarcoidosis. C-reactive protein (CRP) and soluble interleukin-2 receptor (sIL-2R) were increased in the sera. An alternative cause of granulomatous disease was ruled out, and on follow-up, she was diagnosed with sarcoidosis. Two years later, she was hospitalized for fever, anorexia, lymph node tenderness, and erythema nodosum with significant increases in CRP and sIL-2R. After admission, the repetitive axillary lymph biopsy showed the same histological findings as before, but the G-band staining showed clonal abnormalities. Bone marrow biopsy revealed abnormal lymphocytes with petal-like nuclei. Finally, she was diagnosed with malignant lymphoma infiltrating the bone marrow. After CHOP-based chemotherapy, her laboratory data, lymphadenopathy, and clinical findings improved, and she was discharged from the hospital on the 90th day. Careful medical treatment, including genetic analysis of the lymph node, is necessary in patients with sarcoidosis if lymphadenopathy is predominant.