The coagulation parameters factor VII, fibrin monomers, thrombin-antithrombin III (TAT) complexes and fragment 1.2 (F 1.2) were studied in 43 treated and 11 untreated patients (27 males, 27 females age range 19-70 years) with hypertension of moderate severity. The patients included in this study who were treated with antihypertensive drugs were still hypertensive in spite of their treatment. The median F 1.2 concentrations (interquartile range) in the hypertensive patients were more than double those of the reference group: 1.47 (0.79) nmol/l as against 0.74 (0.49) nmol/l (p < 0.0001). Median concentrations of TAT complexes 2.9 (1.7) micrograms/l versus 2.6 (1.6) micrograms/l (p < 0.02) as well as those of fibrin monomers 14.2 (4.6) nmol/l as against 10.6 (2.0) nmol/l (p < 0.01) also were significantly elevated in the hypertensive patients, but to a lesser extent. For factor VII a significant difference was found between males and females. The median factor VII value in the male patients was 137% (32%) compared with 100% (33%) in the male reference group (p < 0.001). In the hypertensive female patients this median value was 147% (36%) in comparison with 139% (60%) in the female reference group (p < 0.01). By the Spearman rank test, no correlations were found between the coagulation parameters and systolic or diastolic blood pressure, age or duration of hypertension. F 1.2 values were correlated with fibrin monomers (r = 0.33, p < 0.03) but not with the other coagulation parameters studied. The elevated F 1.2 values, together with elevated concentrations of TAT complexes and fibrin monomers, are signs of an activated coagulation system in these hypertensive patients.