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PURPOSE To compare the diffusion-weighted imaging of nonthrombosed distensible venous malformations of the orbit with that of other histologically-proven orbital tumors. DESIGN Retrospective case-control study. METHODS Patients with nonthrombosed distensible venous malformations of the orbit and patients with other histologically-proven orbital tumors were selected for chart review. The main outcome measure was the apparent diffusion coefficient of these lesions. RESULTS Sixty-seven patients qualified for chart review; 9 patients had nonthrombosed distensible venous malformations and 58 patients had other histologically-proven tumors. Three of the 9 patients with nonthrombosed distensible venous malformations were initially misdiagnosed as having had solid orbital tumors. The mean apparent diffusion coefficient of distensible venous malformations was 2.80 ± 0.48 × 10-3 mm2/s, whereas the mean apparent diffusion coefficient of other histologically-proven tumors was 1.18 ± 0.39 × 10-3 mm2/s (P < .001). The mean apparent diffusion coefficient ranged from 2.42 to 3.94 × 10-3 mm2/s in the distensible venous malformation group, whereas other histologically-proven tumors ranged from 0.53 to 2.08 × 10-3 mm2/s. Therefore, in this single-institution series, a threshold value of 2.10 × 10-3 mm2/s was 100% sensitive and 100% specific for distensible venous malformations. CONCLUSION Certain nonthrombosed distensible venous malformations can evade diagnostic suspicion and mimic solid orbital tumors on standard magnetic resonance imaging sequences. In this single-institution series, diffusion-weighted imaging effectively distinguished these nonthrombosed distensible venous malformations from other orbital tumors.