Raised oxygen tension in the blood of varicose veins has led to arteriovenous communications being implicated in the pathogenesis of varicose veins. The aim of this study was to compare the oxygen tension of blood from varicose veins with that from normal legs, and to observe the effect of posture on these measurements. Twenty-six subjects (13 normal controls and 13 with uncomplicated varicose veins) had blood sampled from the long saphenous vein or a varicose vein near the ankle. Samples were taken after 30 min in the supine position and repeated after 30 min standing. Samples were also taken simultaneously from the arm. Transcutaneous oxygen measurements of the gaiter skin were performed continuously throughout the experiment. In the supine position, blood from varicose veins had a significantly higher oxygen tension (median = 6.09 kPa) than that from normal veins (median = 4.54 kPa) (P = 0.022). In all subjects, there was a higher oxygen tension in leg vein blood when lying (varicose vein median value = 6.09 kPa, control median value = 4.54 kPa) than while standing (varicose vein median value = 3.93 kPa, control median value = 3.74 kPa) (P less than 0.002, varicose vein group; P = 0.005, normal group). Changes in transcutaneous oxygen tension correlate poorly with changes in venous blood. It is concluded that the theory of arteriovenous anastomoses is only one among a number of possible explanations for the pathogenesis of varicose veins.