Thymidine kinase 1 (TK1) is converting thymidine to thymidine monophosphate, and is related to DNA replication and cell proliferation. The use of the TK1 protein levels as a proliferation marker in malignancies is here summarized. TK1 protein in serum (STK1p) and TK1 expression in tissues were determined by a chemoluminescent dot blot assay and by immunohistochemistry staining, respectively. The expression of TK1 in tumor tissues correlated to pathological stages and clinical grades of carcinomas (ca) of esophagus, lung and in premalignancy of breast ductal ca. STK1p could monitor the out-come of tumor therapy by being correlated to remission [breast ca, non-Hodgkin's lymphoma], relapse [breast ca] and to survival [non-Hodgkin's lymphoma] of patients. In a health screening study of 12,641 persons, STK1p seemed to predict the risk of development of neoplasia related diseases at early stage.