Ascending thrombophlebitis of the superficial leg veins is known to propagate into the deep leg veins and to embolize. In a prospective study we followed up 44 patients with sonographically diagnosed ascending thrombophlebitis into the deep veins (V. saphena magna n = 40, V. saphena parva n = 4). In 15 of 44 cases (34%) thrombosis of the crossing veins was found intraoperatively and 6 of 44 crossings were filled with floating thrombi into the deep vein lumina (14%). Among complications of treatment (11.4%) recurrence of thrombi in the ligated superficial residual vein stump was seen in 2 of 44 cases. One of these patients suffered a symptomatic, non-fulminant pulmonary embolism. The other patient developed a femoral vein thrombosis. 1 patient had an abscess and 1 a seroma of the groin. In 11% of all cases ascending thrombophlebitis diagnosed duplex sonographically was not effective in preventing propagation of thrombi into the deep veins thromboembolism remains a complication of ascending thrombophlebitis.