The purpose of this study was to investigate the occurrence rate of APC resistance (APC-R) with severe preeclampsia in Turkish women. Thirty-two consecutive women having severe preeclampsia were included in the study. Thirty-two healthy pregnant women served as the control group. APC-R assays were performed in the third trimester of pregnancy, and 3 and 9 months after delivery. APC-R was demonstrated in the third trimester, 3 months and 9 months after delivery in 27 (84.4%), 23 (71.9%) and 5 (15.6%) of 32 preeclamptic patients, respectively. APC-R rates were significantly higher in preeclamptic group than in normal pregnant women in the third trimester of pregnancy (p < 0.05). Decreased mean APC activity and also increased APC-R rate was still persisting in preeclamptic group for 3 months after delivery. Nine months after delivery, the mean APC activity and also APC-R rates approached to the normal pregnant women; however, there was a significant difference between both groups (p < 0.05). Our results indicate that acquired APC-R may be a contributory factor in the pathogenesis of preeclampsia. Copyright (C) 2001 S, Karger AG, Basel.