Patients with hereditary angioedema (HAE) have impaired health-related quality of life (HRQoL), but the effect of preventative treatment strategies on HRQoL has not been evaluated. This study was designed to evaluate the effect of routine prevention therapy with nanofiltered C1 inhibitor (C1 INH-nf; human) on the HRQoL of patients with HAE. Thiry-six-item Short Form (SF-36) Version 1.0 questionnaires were administered at the beginning and end of two 12-week treatment periods in this multicenter, randomized, placebo-controlled, crossover study. Patients (n = 22) received intravenous injections of 1000 U of C1 INH-nf or placebo every 3-4 days for 12 weeks and then crossed over to the other treatment arm for a second 12-week period. Patients could receive open-label C1 INH-nf (1000 U) for the acute treatment of angioedema attacks in either arm of the study. Sixteen patients had evaluable SF-36 data. Mean physical component summary scores (PCSs) were 36.41 at baseline, 37.06 at the end of the placebo period, and 43.92 at the end of the C1 INH-nf period. Mean mental component summary scores (MCSs) were 49.90, 44.98, and 54.00, respectively. Least square mean differences (95% confidence intervals) between C1 INH-nf and placebo in norm-based SF-36 scores at the end of each treatment period were 6.55 (1.48, 11.62; p = 0.015) for PCS and 8.70 (1.67, 15.72; p = 0.019) for MCS. In a clinical trial setting, patients with HAE had significantly better HRQoL after 12 weeks of C1 INH-nf for routine prevention compared with acute treatment of individual angioedema attacks in the absence of routine prevention while on placebo. This study was a part of the clinical trial NCT01005888 registered in www.clinicaltrials.gov.