In 31 patients with unilateral primary varicose veins the resting interstitial pressure in the deep posterior compartment of the leg was measured in three groups: I, asymptomatic; II, symptomatic; III, symptomatic with trophic changes. Spinal anesthesia was induced in all patients, and polytetrafluoroethylene (Teflon) catheters were inserted into the deep posterior compartment of both legs. The interstitial pressure was recorded with a bioelectronic strain-guage pressure monitor with the patient in the horizontal position with complete muscle relaxation. In control legs (n = 31) mean pressure was -2.48 mm Hg (range -5 to 1). However, the measurement in affected legs (n = 31) was significantly higher at 4.61 mm Hg (range 1 to 12). Results demonstrate the following: (1) Primary varicose veins (an exclusively epifascial abnormality) were consistently associated with an increased resting interstitial pressure of the subfascial tissues in the supramalleolar area of the leg. (b) The subfascial resting interstitial pressure was elevated in all stages of varicose veins, even those with short duration of the disease and with no symptoms at all. (c) In all three groups there was a significant increase in the subfascial interstitial pressure between the affected and contralateral extremity. In patients with trophic changes (group III) this increase was significantly higher than in the remaining groups, although there was no significant difference between the increased pressure of the remaining groups (groups I & II).