Inferior vena cava agenesis is a rare congenital venous malformation, with a prevalence ranging from 0.0005 to 1% of the world's population. The present study aims to describe a case of agenesis of the inferior vena cava associated with deep vein thrombosis (DVT) in a young patient and the difficulty in maintaining adequate anticoagulation. We report the case of a 22-year-old woman who had her first thrombotic event as a child, in the renal vein, and was anticoagulated. At age 20, she had a thrombotic event in the iliac vein and went back on anticoagulation, but with adequate control of coagulation, the patient decided when to stop and return to anticoagulation without medical advice. The diagnosis of agenesis was confirmed by tomography and phlebography. Currently, she was maintained on anticoagulation and warned about the risks of the disease and inadequate anticoagulation.