Hereditary angioedema (HAE) is an inherited deficiency of C1 esterase inhibitor (C1 inh). The two types of genetic C1 inh deficiency are type I, which is quantitative, and type II, which is functional. For the purpose of the present study, four HAE patients were selected. None of them had received any androgenic therapy. The group included three type I and one type II cases. All patients that entered the protocol received danazol, 400 mg/day for 14 days. The complement system was evaluated by monitoring C4, hemolytic complement 50% (CH50), circulating immune complexes (CIC), and antigenic and functional C1 inh during the study. The level of complement factors at the beginning and at the end of this period demonstrated a statistically significant increase in C4 and CH50 and the disappearance of CIC, while C1 inh remained unmodified. These results suggest that the therapeutic effect of danazol may have two mechanisms of action: (1) promotion of C4 synthesis by anabolic effect resulting in an improvement of the complement system with the disappearance of CIC and (2) a minor increase in C1 inh level primarily due to the lack of its consumption.