<h3>Introduction</h3> Hereditary angioedema (HAE) is a potentially fatal disease characterized by recurrent, often disabling swelling. A phase 2 study (ISIS 721744-CS2, NCT04030598) in patients with HAE treated with donidalorsen reported a 90% reduction in HAE attacks and lower adverse event (AE) rates compared with placebo (71% vs 83%). Here we report the results of the open-label extension of the phase 2 study (ISIS 721744-CS3, NCT04307381). <h3>Methods</h3> Patients completing the phase 2 study were eligible for enrollment. The on-treatment study period consisted of fixed (Weeks 1–13, donidalorsen 80 mg subcutaneously every 4 weeks) and flexible treatment periods (Weeks 17–53). In the flexible dosing period, patients continued 80 mg every 4 weeks, or switched to 80 mg every 8 weeks or 100 mg every 4 weeks. <h3>Results</h3> Seventeen patients with HAE (mean age 38.9 years, 64.7% female) were enrolled. No serious AEs or patient discontinuation due to AEs were reported. Patients were HAE attack-free for 99.6% (95% CI, 99.3%–99.9%) of on-treatment study days. The overall mean HAE attack rate for the on-treatment period starting from Day 1 was 0.08/month, 94.6% lower than the phase 2 run-in rate (2.7/month). Patients receiving 80 mg every 8 weeks (n=8) had a 76% reduction in HAE attacks (mean 0.28/month) from baseline (n=5, HAE attack-free; n=3, reverted to 80 mg every 4 weeks). <h3>Conclusion</h3> Results of this study demonstrated sustained reduction in HAE attack rate and no new safety signals with donidalorsen treatment, confirming prior phase 2 study findings and supporting its continued development.