Capillary hydrostatic pressure has been calculated in normal pregnancy and preeclampsia. In humans, capillary hydrostatic pressure cannot be measured directly but may be calculated when the colloid osmotic pressure in plasma and interstitial fluid and interstitial fluid hydrostatic pressure are known (Starling equation). New methods have made it possible to measure the interstitial fluid colloid osmotic pressure and interstitial fluid hydrostatic pressure. In the present study interstitial fluid was collected from the subcutaneous tissue by implanted wicks (wick method), and interstitial fluid colloid osmotic pressure was determined. Interstitial fluid hydrostatic pressure was recorded by the wick-in-needle technique. Capillary hydrostatic pressure was calculated in 10 women in the first trimester and 10 in the third trimester of normal pregnancy, in 15 patients with mild preeclampsia, and in 13 women with severe preeclampsia. In normal pregnancy, capillary hydrostatic pressure increased by about 30% between the first and third trimesters. In mild preeclampsia, capillary hydrostatic pressure values did not differ significantly from those in the third trimester of normal pregnancy. However, in severe preeclampsia capillary hydrostatic pressure was significantly lower (40%) than in mild preeclampsia. Whether the low capillary hydrostatic pressure is caused by the severe general vasospasm seen in this condition or is a secondary event is unknown.