Hereditary angioedema (HAE) is a rare disease caused by C1INH gene mutations, which leads to a deficiency or dysfunction of C1 inhibitor (C1 INH), resulting in recurrent episodes of severe and potentially life-threatening edema. To evaluate the efficacy and safety of repeat use of nanofiltered C1 esterase inhibitor (human) (C1 INH-nf) for the short-term treatment of HAE attacks. In this open-label study, patients received C1 INH-nf, 1,000 U intravenously, for the treatment of HAE attacks. Efficacy end points included the proportion of attacks with unequivocal relief of the defining symptom within 1 and 4 hours after receiving study drug and time to beginning of relief of the defining symptom. Safety was monitored through adverse event reporting, vital signs measurements, and laboratory testing. A total of 113 patients were enrolled in the study from September 21, 2006, through March 31, 2009, and received 885 doses of C1 INH-nf. A total of 609 HAE attacks were treated with C1 INH-nf, and the numbers of attacks achieving unequivocal relief of the defining symptom within 1 and 4 hours after the start of the first dose were 412 (68%) and 529 (87%), respectively. Of 101 patients treated for an attack during the study period, 80 achieved unequivocal relief of their first attack within 4 hours after study medication (response rate, 79%); median time to the beginning of unequivocal relief was 0.75 hour. C1 INH-nf was safe and well tolerated. This open-label study demonstrates the efficacy and safety of C1 INH-nf for short-term treatment of HAE attacks. clinicaltrials.gov Identifier: NCT00438815.