From August 1989 to October 1990, 83 pregnant Chinese women were the subjects for measuring the levels of plasma functional antithrombin III (AT III) activity. The correlations of AT III activity with perinatal outcome and the changes in maternal hepatorenal function were analyzed. The population was divided into four groups: Group I (n = 30), normal pregnancies; Group II (n = 23), mild pre-eclampsia; Group III (n = 26), severe pre-eclampsia; and Group IV (n = 4), eclampsia. The results demonstrated that: 1) AT III activity decreased with the severity of toxemia (p < 0.001), 2) AT III activity correlated with the degree of perinatal outcome and maternal morbidity, and 3) reduction of AT III activity correlated with impairment of maternal hepatorenal function. In conclusion, plasma AT III activity is a valuable parameter in the evaluation of toxemia.