Starling forces (interstitial fluid pressure, plasma- and interstitial fluid colloid osmotic pressures) were measured in the subcutaneous tissue of legs in 22 patients with deep venous thrombosis (DVT). For comparison, measurements were obtained in the contralateral leg and in healthy controls as well. Interstitial fluid pressure (Pif) was measured with the 'wick-in-needle' technique and we found a significantly higher pressure in limbs with DVT compared to contralateral limbs and controls. Interstitial fluid was collected from subcutaneous tissue with the 'wick technique'. Colloid osmotic pressure was measured in wick interstitial fluid (COPif) and plasma (COPpl) with an osmometer. A significant reduction of COPif was found in limbs with DVT compared to the contralateral side and controls. The COPpl was also significantly reduced in patients compared to controls. Saphenous vein pressure (Pv) measured at the ankle was significantly increased in the thrombosed limb compared to the contralateral. We conclude that the main cause of subcutaneous oedema in DVT of the lower limbs is increased transcapillary filtration caused by a rise in Pv. This leads to a fall in COPif and rise in Pif which act as oedema-preventing mechanisms.