The aim of this study is to assess the efficacy of C1-esterase inhibitor in preventing hereditary angioedema attacks when it is administered under the skin of subjects with hereditary angioedema. The safety of C1-esterase inhibitor will also be assessed. Each subject will enter a run-in period of up to 8-weeks. Subjects who complete the run-in period and who are eligible will then enter the treatment phase which comprises two sequential treatment periods. In the treatment phase, subjects will be randomized to one of four arms consisting of treatment with low- or higher-volume C1-esterase inhibitor in one treatment period and treatment with low- or higher-volume placebo in the other treatment period. The study will measure the number of hereditary angioedema attacks that subjects experience while receiving each treatment.