Background: We aimed to explore the frequency of occult cancer in patients with deep-vein thrombosis (DVT) at a general hospital in Mexico City. Materials and Methods: From March 2012 to February 2015, all patients with primary DVT of lower extremities attended in the emergency department of our hospital were studied. Initially, all patients were evaluated with clinical history, physical examination, basic laboratories, abdominal ultrasound, chest X-ray, and duplex venous ultrasonography. In a case-by-case approach, if necessary, computed tomography, endoscopy, colonoscopy, and tumor markers were done. Results: From 182 patients with primary DVT, 30 (16.5%) presented occult cancer: Thirteen males and 17 females, with an average age of 61 years. In males, prostate cancer prevailed (6/13, 46%); meanwhile, in females, pelvic gynecologic cancers predominated (7/17, 41%). Conclusion: Our results suggest that in Mexican patients with primary DVT, occult cancer is frequent.