This is the first case report of acute sarcoidosis with increased serum thymidine kinase (TK) activity. A 43-year-old male presented fever, swelling of parotid glands, lymphadenopathy, and peripheral neuropathy. Sarcoidosis was pathologically diagnosed by lung and parotid gland biopsy. His serum TK, which was increased to 11.2 U/l at diagnosis (normal <5 U/l), normalized after glucocorticoid therapy. Serum TK has been considered as a good marker of the proliferative activity of various types of neoplasms. Its rise in sarcoidosis has, however, not been described. Because acute sarcoidosis sometimes resembles malignant lymphoma, the possible rise of serum TK in sarcoidosis may be worthy of note.