Purpose: To identify the baseline patient characteristics that predict who will benefit from pharmacomechanical catheter-directed thrombolysis (PCDT) of acute iliofemoral deep vein thrombosis (DVT). Materials and Methods: In the Acute Venous Thrombosis: Thrombus Removal with Adjunctive Catheter-Directed Thrombolysis (ATTRACT) multicenter randomized trial, 381 patients with acute iliofemoral DVT underwent PCDT and anticoagulation or anticoagulation alone. The correlations between baseline factors and venous clinical outcomes were evaluated over 24 months using post hoc regression analyses. Interaction terms were examined to evaluate for differential effects by treatment arm. Results: Patients with clinically severe DVT (higher baseline Villalta score) experienced greater effects of PCDT in improving 24-month venous outcomes, including moderate or severe postthrombotic syndrome (PTS) (odds ratios [ORs] and 95% confidence intervals [CIs] per unit increase in the baseline Villalta scores were as follows: for PCDT, OR, 1.08 [95% CI, 1.01–1.15]; for control, OR, 1.20 [95% CI, 1.12–1.29]; Pinteraction = .03), PTS severity (between-arm differences in the Villalta [Pinteraction = .004] and Venous Clinical Severity Scale [VCSS] [Pinteraction = .002)] scores), and quality of life (between-arm difference in the Venous Insufficiency Epidemiological and Economic Study Quality of Life score; Pinteraction = .025). Patients with previous DVT had greater effects of PCDT on 24-month PTS severity than those in patients without previous DVT (mean [95% CI] between-arm difference in the Villalta score, 4.2 [1.56–6.84] vs 0.9 [−0.44 to 2.26], Pinteraction = .03; mean [95% CI] between-arm difference in the VCSS score, 2.6 [0.94–4.21] vs 0.3 [−0.58 to 1.14], Pinteraction = .02). The effects of PCDT on some but not all outcomes were greater in patients presenting with left-sided DVT (Villalta PTS severity, Pinteraction = .04; venous ulcer, Pinteraction = .0499) or a noncompressible popliteal vein (PTS, Pinteraction = .02). The effects of PCDT did not vary by sex, race, ethnicity, body mass index, symptom duration, hypertension, diabetes, or hypercholesterolemia. Conclusions: In patients with acute iliofemoral DVT, greater presenting clinical severity (higher baseline Villalta score) and a history of previous DVT predict enhanced benefits from PCDT.