The cause(s) of the low plasma antithrombin III activity in women with preeclampsia is unknown. The purpose of this study was to evaluate the relationship between plasma antithrombin III activity and both clotting parameters (platelet count and fibrinopeptide A) and renal parameters (serum creatinine and uric acid concentration). Forty-seven preeclamptic women were studied within the 2-day interval before delivery. Creatinine did not correlate significantly with any of the other study parameters. However, there were significant correlations between antithrombin III activity and platelet count (r = 0.53, P less than .005) and between antithrombin III and fibrinopeptide A (r = -0.29, P less than .05). The correlation between platelet count and fibrinopeptide A was not significant. These findings support the hypothesis that antithrombin III consumption by fibrin generation is initiated by platelet activation, and is at least one cause of the low activity level known to occur in women with preeclampsia.