Cell-free DNA (cfDNA) is a fragment of DNA circulating in the blood, and its concentration is often elevated in cancer patients. To investigate the relationships between serum cfDNA concentration and clinical characteristics, including prognosis, we measured serum cfDNA concentration in 114 newly diagnosed lymphoma patients. The cfDNA concentrations in diffuse large B cell lymphoma (DLBCL) (62.5 ng/mL) and follicular lymphoma patients (51.6 ng/mL) were significantly elevated compared to healthy individuals (7.5 ng/mL, P < 0.001). In DLBCL, patients with elevated serum cfDNA (> 38.9 ng/mL) at diagnosis had significantly shorter time-to-progression compared to those without (P = 0.033). The addition of cfDNA concentration to the international prognostic index showed improved predictive power for time-to-progression. Moreover, cfDNA added significant prognostic value to other inflammatory markers such as B symptoms and sIL2R. There was a trend towards shorter progression-free survival and overall survival in patients with elevated cfDNA. Furthermore, B symptoms (P = 0.038), bulky masses (P = 0.031), non-GCB subtype (P = 0.012), and serum sIL-2R levels > 2,000 U/mL (P = 0.012) were associated with higher cfDNA levels. Our study showed that serum cfDNA concentration at diagnosis was associated with certain clinicopathological characteristics, and may be predictive of survival outcomes in DLBCL patients.