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Acquired angioedema due to C1-inhibitor deficiency (C1-INH-AAE) is a rare disease that can be diagnosed via complement testing. It often accompanies lymphoproliferative underlying diseases. Our study aimed to examine if there is a connection between complement parameters and the clinical symptoms of C1-INH-AAE, and, in case of a known underlying disease, its activity. The other question is how a connection, if proven, could help in the development of the therapeutic strategy of C1-INH-AAE patients. In the past 30 years, out of the 3938 patients sent to the Angioedema Center with angioedema symptoms, we have diagnosed C1-INH-AAE in 19 cases. An underlying disease was diagnosed in 15 patients. Most often lymphoma (6/19 patients) and monoclonal gammopathy of undetermined significance (6/19 patients) were found. Angioedema specific long-term prophylaxis did not result in an improvement in neither the frequency of the attacks nor in the complement parameters. A connection has been found between the presence and activity of any underlying disease, the frequency of the angioedema attacks and the decreased level of proteins of the complement system. Decreasing complement parameters warn about the appearance or the worsening of the underlying disease. The treatment of the underlying disease brings improvement in the complement parameters. Rituximab treatment reduced the number of attacks or completely made them disappear, and we experienced positive changes in complement parameters. Complement parameters supported the long-term efficacy of rituximab treatment for C1-INH-AAE. The change in complement parameters predict the relapse of the underlying disease, and it is a good indicator for the prediction of angioedematous attacks. In C1-INH-AAE, it is essential to examine the patients for underlying diseases, and to regularly follow up the patient's complement parameters., Competing Interests: The authors declare the following conflict of interests: H. Farkas has received honoraria and travel grants from CSL Behring, Shire/Takeda, Swedish Orphan Biovitrum, Octapharma, Kalvista and Pharming; and/or served as a consultant for these companies and has participated in clinical trials/registries for BioCryst, CSL Behring, Pharming, Pharvaris and Shire. Zs. Balla has participated in clinical trials of CSL Behring, Pharvaris and Takeda. The other authors have declared that no conflict of interest exists., (© 2022 The Author(s).)