The aim of this study was to evaluate the CEA, CA 125, CA 15-3, CA 19-9 and AFP tumor markers as screening tests for neoplasms in idiopathic deep venous thrombosis (DVT). The hospital records of 500 patients with confirmed diagnosis of DVT by lower limb Doppler ultrasonography were assessed in retrospective longitudinal and quantitative study in Hospital de Base Medicine School, São José do Rio Preto in the period from January 2005 to March 2010. An investigation of tumor markers had been randomly requested for 167 of these patients. There was significant difference for the CEA marker in patients with neoplasms detected after idiopathic DVT (124.42 ± 474.82) compared to other patients in this group (1.64 ± 1.41; p = 0.0017, 95% confidence interval (CI): -198.76 to -46.788) and for the CA 15-3 marker (57.5 ± 0.707 in patients with cancer and DVT and 18 ± 11.015 in patients with only idiopathic DVT; p = 0.0019; 95% CI: -58.844 to -20.156). Only the CA 15-3 marker proved to be noticeably sensitive for screening in the statistical tests. Cancer was diagnosed in 32% of 28 the patients who had elevated levels of at least one of the markers. All patients without diagnosis of malignancy had serum levels of tumor markers within the reference values. An investigation of tumor markers proved useful both in the exclusion of malignancies and to indicate screening utilizing other more specific examinations for patients with a high probability of neoplastic syndrome.