Objectives: The aim of this study was to evaluate the diagnostic performance of noncontrast magnetic resonance imaging utilizing sampling perfection with application optimized contrasts using different flip angle evolutions (SPACE) in detecting deep venous thrombus (DVT) of the lower extremity and evaluating clot burden. Materials and Methods: This prospective study was approved by the institutional review board. Ninety-four consecutive patients (42 men, 52 women; age range, 14-87 years; average age, 52.7 years) suspected of lower extremity DVT underwent ultrasound (US) and SPACE. The venous visualization score for SPACE was determined by 2 radiologists independently according to a 4-point scale (1-4, poor to excellent). The sensitivity and specificity of SPACE in detecting DVT were calculated based on segment, limb, and patient, with US serving as the reference standard. The clot burden for each segment was scored (0-3, patent to entire segment occlusion). The clot burden score obtained with SPACE was compared with US using a Wilcoxon test based on region, limb, and patient. Interobserver agreement in assessing DVT (absent, nonocclusive, or occlusive) with SPACE was determined by calculating Cohen kappa coefficients. Results: The mean venous visualization score for SPACE was 3.82 +/- 0.50 for reader 1 and 3.81 +/- 0.50 for reader 2. For reader 1, sensitivity/specificity values of SPACE in detecting DVT were 96.53%/99.90% (segment), 95.24%/99.04% (limb), and 95.89%/95.24% (patient). For reader 2, corresponding values were 97.20%/99.90%, 96.39%/99.05%, and 97.22%/95.45%. The clot burden assessed with SPACE was not significantly different from US (P > 0.05 for region, limb, patient). Interobserver agreement of SPACE in assessing thrombosis was excellent (kappa = 0.894 +/- 0.014). Conclusions: Non-contrast-enhanced 3-dimensional SPACE magnetic resonance imaging is highly accurate in detecting lower extremity DVT and reliable in the evaluation of clot burden. SPACE could serve as an important alternative for patients in whom US cannot be performed.