Eighty symptomatic ambulatory outpatients with acute deep vein thrombosis were evaluated with ascending contrast venography and ultrasonic imaging to determine the origin and distribution of thrombosis and to analyze clinical risk factors. Isolated calf vein thrombosis was present in 42.5 percent of the limbs, combined calf and proximal deep vein thrombosis in 47.5 percent, and isolated proximal thrombosis in 10 percent of the limbs. Discontinuity of thrombus was present in 55 percent, whereas 45 percent exhibited a continuous column of thrombus. The results of this study indicate that in the ambulatory outpatient population, acute deep vein thrombosis begins segmentally in the calf and proximal vessels and frequently coalesces into a continuous column of thrombus over several days. We believe that all cases of acute deep vein thrombosis should be treated and patients with evidence of previous acute deep vein thrombosis should be closely monitored for possible recurrences.