Extracellular DNA in the plasma or serum of cancer patients has been recently proposed as a source of analyzable cancer-related gene sequences (qualitative approach). Furthermore, patients with different tumor types show high levels of cell-free circulating DNA both in plasma and serum (quantitative approach) at the time of surgery. Our aim was to verify whether the level of cell-free DNA in plasma might help in detecting recurrences during follow-up of colorectal cancer (CRC) patients. We studied 70 patients undergoing surgery for primary CRC. Plasma samples were obtained at the time of surgery and during follow-up. The cell-free circulating DNA in plasma was quantified by the Dipstick Kit method. At the time of surgery, in all patients, cell-free DNA levels in plasma were about 25 times higher in comparison with 20 healthy donors. In contrast, the carcinoembryonic antigen (CEA) value of this cohort of patients was altered in only about 37% of cases. During follow-up, cell-free DNA levels decreased progressively in tumor-free patients, while it increased in those developing recurrences or metastases. The results were further supported by qualitative analysis of circulating tumor-specific DNA, such as K-Ras mutations and p16(INK4a) promoter hypermethylation. These preliminary data confirm that plasma tumor DNA levels (i) are significantly higher in patients with CRC, (ii) decrease progressively in the follow-up period in tumor-free patients, and (iii) increase in patients with recurrence or metastasis. We suggest, therefore, that the quantification of plasma cell-free DNA might represent a useful tool for monitoring of CRC and, prospectively, for identifying high-risk individuals.