['Iliofemoral deep vein thrombosis (DVT) is associated with severe postthrombotic morbidity when treated with anticoagulation alone. Catheter-directed thrombolysis (CDT), with or without the addition of mechanical techniques, is increasingly recommended for patients with iliofemoral DVT, although its effect on postthrombotic syndrome is not established. This study examined the correlation of residual thrombus with postthrombotic syndrome after catheter-based attempts at thrombus removal in patients with iliofemoral DVT.', 'Seventy-one consecutive patients with iliofemoral DVT were treated with CDT. Pretreatment and posttreatment phlebograms were evaluated for quantity of residual thrombus by physicians blinded to clinical patient outcomes. Postthrombotic syndrome was assessed using CEAP and Villalta scores by examiners blinded to phlebographic results. Patients were grouped by the amount of residual thrombus in treated vein segments (group 1, ≤50%; group 2, >50%). Clinical score and postthrombotic outcomes were plotted vs residual thrombus.', 'Sixty-three of 71 patients completed CEAP and Villalta analyses. Groups 1 and 2 had median CEAP scores of 1 and 4 (P = .025) and mean Villalta scores of 2.21 and 7.13, respectively (P = .011). There was a direct and significant correlation of clinical class of CEAP with residual thrombus (R(2) = .74; P = .004) and a direct linear correlation of Villalta score with residual thrombus (R(2) = .61; P = .0014).'] Does postthrombotic morbidity correlate with residual thrombus following catheter-directed thrombolysis for iliofemoral deep vein thrombosis?