This study of 51 patients aims to define the 'normal' range of reflux in the deep veins and the incidence of pathological reflux in patients with the post-thrombotic syndrome (PTS). 'Normal' limbs, limbs with simple varicose veins and limbs with post-thrombotic syndrome have been studied using a standardized technique for descending phlebography, supine and with a controlled Valsalva. The 'normal' range of reflux has been found to be grade 0-2. The incidence of pathological reflux in patients with proven post-thrombotic damage to the deep veins is 31 per cent.