In patients with a venous outflow obstruction following iliofemoral deep vein thrombosis stenting of the venous tract to prevent or alleviate postthrombotic syndrome is applied with increasing frequency. The impact of the quality of anticoagulant therapy with vitamin K antagonists (VKAs) on the development of in-stent thrombosis is currently unknown.To determine the association between the quality of postinterventional VKA treatment and the occurrence of in-stent thrombosis.Seventy-nine patients with iliofemoral and/or caval venous stent placement for obstruction of the venous outflow were included in this study. All patients received postinterventional VKA. The quality of VKA anticoagulant therapy was expressed as the time within therapeutic range (TTR) calculated using the linear interpolation method and as the proportion of International Normalized Ratio (INR) values < 2.0. In-stent thrombosis was assessed by the use of duplex ultrasound. Survival analysis (Kaplan-Meier curves, Cox regression) was used to analyze the data.In-stent thrombosis developed in 16 patients (20.3%). The total population had a mean TTR of 64.0% (±19.0) and a mean proportion of INR values < 2.0 of 11.6% (±12.0). Overall, a TTR < 49.9% was associated with an increased risk of in-stent thrombosis. The multivariable adjusted analysis showed a hazard ratio (HR) of 0.96 (95% confidence interval [CI], 0.92-0.99; P = .02) per 1% increase in TTR. The proportion of INR values < 2.0 had no significant association with the occurrence of in-stent thrombosis: HR 0.98 (95% CI, 0.91-1.06; P = .66).We conclude that the quality of anticoagulant treatment reflected in the TTR following a venous stenting procedure is an important independent determinant for the risk of in-stent thrombosis. The role of anticoagulant treatment for the prevention of in-stent thrombosis following stenting procedures therefore merits further research.