Background: The first subcutaneous (SC) C1-esterase inhibitor concentrate (C1-INH) was approved by the U.S. Food and Drug Administration in June 2017 as routine prophylaxis to prevent hereditary angioedema attacks in adolescents and adults at a dose of 60 IU/kg twice weekly based on the phase III Clinical Study for Optimal Management of Preventing Angioedema With Low-volume Subcutaneous C1-Inhibitor Replacement Therapy (COMPACT) trial.Objective: This article aimed to evaluate the relationship of the C1-INH (SC) dose regimens tested in the COMPACT trial (40 IU/kg and 60 IU/kg twice weekly) and the occurrence of adverse events (AEs).Methods: Patients were instructed to record any AEs in their e-diary daily. Safety and tolerability were assessed based on reported AEs, including injection-site reactions (ISRs); unsolicited AEs (AEs other than ISRs); serious AEs; thrombotic, thromboembolic, anaphylactic, hypersensitivity, sepsis, and bacteremia events; inhibitory antibodies to C1-INH; and clinically significant abnormalities in laboratory assessments. Information on ISRs was specifically solicited.Results: No relationship between the dose of C1-INH (SC) and the incidence of ISRs or unsolicited AEs was observed. The proportion of injections followed by at least one ISR was 12% with C1-INH (SC) 40 IU/kg versus 5% with 60 IU/kg and 6% with placebo. No ISRs were serious or led to treatment discontinuation, and all resolved. There were no anaphylaxis, thromboembolic, sepsis, or bacteremia events reported during treatment with C1-INH (SC). All hypersensitivity AEs were nonserious, and the majority were assessed as being unrelated to treatment. No inhibitory antibodies to C1-INH were observed.Conclusion: C1-INH (SC) is safe and well tolerated with no dose-dependent safety concerns, as demonstrated in the COMPACT trial.