Deep venous valvular incompetence (DVI) of primary, non-thrombotic origin has received little interest compared with secondary, postthrombotic DVI. In this study both limbs of 308 patients with untreated or surgically treated varicose veins in at least one limb were examined with Doppler ultrasound for signs of deep vein reflux. Ascending venography was used to exclude limbs with secondary DVI and descending venography to assess valve function. Doppler examination disclosed reflux in 3.5% of the "normal" limbs but in none of the limbs without recurrent varicose veins. A high incidence of primary DVI was found in limbs with untreated varicose veins (20.6%, p less than 0.001) and in those with recurrent or residual varicose veins (42.9%, p less than 0.001), compared with that in "normal" limbs. Popliteal vein reflux was observed in 14% and 37% in these two groups and a combination of femoral and popliteal reflux in 4% and 16%. Reflux of grade 2 or more at descending venography was noted in 43 of 78 limbs with Doppler evidence of reflux. Primary DVI is a common condition in patients with varicose veins. Further studies are needed to determine its clinical significance.