PURPOSE: We studied whether subjective impression of vein size is a valid means of assessment during sonographic evaluation for deep vein thrombosis. METHODS: Diameter was assessed at 5 predetermined venous segments on 975 legs of 721 patients referred for evaluation for lower extremity deep vein thrombosis. The sonographer recorded a subjective assessment of whether the venous segment was enlarged, normal, or narrowed based on a visual impression of sonographic images obtained without compression. This subjective impression was then compared with the absolute measurements of vein diameter and vein diameter:artery diameter ratio at each segment. RESULTS: A significant correlation was found between the subjective characterization of vein size as enlarged, normal, and narrowed and both the absolute vein diameter and the vein:artery ratio at all 5 segments. CONCLUSIONS: Vein size can be reliably categorized by the subjective impression of experienced sonographers. Therefore, actual measurement of the vein diameter is not necessary in the evaluation for deep vein thrombosis in the majority of patients.