Seven patients with hereditary angioedema (HAE) were studied to understand further how physical exercise may induce attacks. The most pronounced differences between patients and controls, however, were independent of the controlled bicycle run (mean values in patients/ controls); C4(g/L): 0.12/0.28 (P = 0.0122); C4bc (AU/ml): 137.0/18.0 (P = 0.0002); C4d (mg/mL): 5.03/2.35 (P = 0.0004); C3bc (AU/ml): 8.4/6.3 (P = 0.0049); C3a (AU/ml): 11.1/5.6 (P = 0.0102). The ratio C4bc to C4 was 1141 versus 64. Consequently, a substantial part of the low amount of C4 left in HAE patients consists of activation products, and the authors show for the first time that a mild but significant activation of C3 occurs in HAE. The two HAE patients treated with danazol had values of C1-INH function and antigen, C4, and C2 in-between those of normal and untreated patients, and lower levels of split products from C4 and high molecular weight kininogen than untreated patients. As a result of the exercise, fibrinolysis increased significantly in both patients and controls, while C1/C1-INH complexes rose significantly only in the five HAE patients without treatment when compared to the seven controls (P = 0.0089). This study thus suggests that complement activation is enhanced in untreated HAE patients following physical stress.