The long-term sequelae of acute deep-vein thrombosis were examined prospectively in a series of 61 patients. The location and extent of the involvement were established by either contrast or isotope phlebography. The mean age of the patients was 47 years, with the duration of follow-up averaging 39 months (range, one to 144 months). Pain and/or swelling was noted in 67% of the patients. Pigmentation developed in 15 limbs (23%). Ulceration developed in three patients. The most important factor with regard to prognosis appeared to be the status of the distal deep veins. If these were found to be patent and competent by ultrasonic velocity studies, the long-term outlook was good, both with regard to symptoms and the development of pigmentation. Only 8% of those limbs with normal distal veins were found to have pigmentation during follow-up, as compared with 40% in those with either occlusion or incompetence of the valves.