Patients with venous thromboembolism (VTE) and active cancer are often excluded from clinical trials of anticoagulant therapy because of short life expectancy, inability or unwillingness to attend for regular laboratory monitoring during vitamin K antagonist therapy, or contraindications to therapy. As such, treatment regimens based on the results from clinical trials might not be suitable for many VTE patients with cancer. 'Registro Informatizado de la Enfermedad TromboEmbólica' (RIETE) was initiated in March 2001 to prospectively record the current clinical management of patients with symptomatic, objectively confirmed, acute VTE in Spanish hospitals. This chapter will summarize the most important findings published to date regarding the association between VTE and cancer.