- Objectives: To evaluate circulating cell-free DNA (CFD) measured by a simple fluorescent assay as a biomarker of breast cancer. Methods: We enrolled 38 patients with breast cancer before surgery, two patients with noncancerous breast lesions, nine patients after surgery, 16 healthy participants, and 29 control women admitted to the hospital emergency ward and released without hospitalization. CFD levels were measured by a direct fluorescence assay. Results: Presurgery patients with cancer had elevated CFD levels (1,010 ± 642 ng/mL), which were higher than those measured in the healthy control group (395 ± 248 ng/mL, P < .001), the noncancer breast lesion group (386 ± 40 ng/mL), the nonhospitalized control group (492 ± 193 ng/mL, P < .001), and the postsurgery cancer group (398 ± 162 ng/mL, P < .01). The area under the receiver operating characteristic curve of the presurgery vs healthy patient group was 0.83. CFD levels correlated with tumor size ( P = .03, ρ = 0.36), nodal involvement ( P = .0003, ρ = 0.56), and TNM stage ( P = .0002, ρ = 0.56). All patients with axillary node involvement had a CFD value greater than 600 ng/mL. Conclusions: CFD measured using a simple fluorometric assay has shown good correlation to stage and enhanced sensitivity to locally advanced disease. A large prospective study is warranted to evaluate if inclusion of this method as a decisive marker before mammography is advantageous.