Serum thymidine kinase 1 (TK1) is a sensitive marker of tumor cell proliferation. TK1 has been reported as a reliable prognostic factor in solid tumors and chronic lymphocytic leukemia, but has not yet been tested in large populations of patients with non-Hodgkin lymphoma. In this study, the prognostic significance of TK1 levels was assessed in 170 prospectively enrolled patients with previously untreated follicular lymphoma (FL). The TK1 level at the time of treatment initiation was shown to correlate with the clinical stage, Follicular Lymphoma International Prognostic Index (FLIPI) score, β(2)-microglobulin level, lactate dehydrogenase level and B symptoms. No correlation was found with FL grade or Ki-67 proliferation index. Cox regression analysis identified high TK1 levels (≥ 15I U/L) as a prognostic factor for overall survival (hazard ratio 2.91, p = 0.019) and progression-free survival (hazard ratio 1.94, p = 0.022) independent of FLIPI score variables. Thus, TK1 levels may help to refine risk assessment in the modern immunotherapy era.