Thymidine kinase (TK) is a cellular enzyme which is involved in a "salvage pathway" of DNA synthesis. It is activated in the G1/S phase of the cell cycle, and its activity has been shown to correlate with the proliferative activity of tumor cells. Additionally, certain viruses are able to induce cellular TK production and activity. Clinical studies have reported elevated serum TK levels in a variety of neoplasias. The majority of these studies concerned hematologic malignancies. TK seems to be a useful marker in non-Hodgkin's lymphoma, where it correlates with clinical staging and provides significant prognostic information on (progression-free) survival. Preliminary results in acute myeloid leukemia indicate that pretreatment serum TK values may predict the response to the first induction chemotherapy. Moreover, serum TK appears to have some clinical value in such solid tumors as prostate cancer, breast cancer, and small-cell lung cancer, whereas it is not a reliable marker of non-small-cell lung cancer and brain tumors.