Fifty-eight legs were investigated by descending phlebography using a standard Valsalva maneuver in 38 patients with varicose veins who were suspected of having primary long saphenous vein incompetence on the basis of clinical examinations, tourniquet tests, and Doppler sonography. In 21 (36%) of 58 legs the clinical diagnosis was confirmed; in the rest, there was no evidence of long saphenous vein incompetence. Descending phlebography using a standard Valsalva maneuver is a reliable method of demonstrating long saphenous vein incompetence and should be performed before surgical treatment in all patients in whom there is clinical doubt about the presence or absence of this condition.