Fever is a presenting sign in some patients with acute deep venous thrombosis (DVT), but its influence on outcome has not been thoroughly investigated. RIETE is an ongoing, international, observational registry of consecutive patients with symptomatic, objectively confirmed, acute venous thromboembolism. The aim of the present study was to assess the prevalence of fever in patients with acute DVT, and to compare their outcome during the first month of therapy, according to the presence or absence of fever. As of September 2009, 14,480 patients with symptomatic DVT have been enrolled in RIETE. Of these, 707 (4.9%) had fever at presentation. During the 30-day study period, 448 patients (3.1%) died, 171 (1.2%) developed DVT recurrences, 376 (2.6%) had pulmonary embolism, and 384 (2.6%) had a major bleeding. Patients initially presenting with fever had a higher mortality (5.8% vs. 2.9%; odds ratio: 2.6; 95% CI 1.9-3.5) than those without fever. Among the causes of death, pulmonary embolism (0.7% vs. 0.1%) and infection (1.1% vs. 0.3%) were significantly more common in patients presenting with fever. Multivariate analysis confirmed that DVT patients with fever had an increased mortality (hazard ratio: 2.00; 95% CI 1.44-2.77) irrespectively of the patient's age, body weight, and risk factors for VTE. Fever is not uncommon in patients with DVT, and carries a worse outcome.