The aim of the study was to evaluate the concentration and activity of C1 esterase inhibitor (C1 INH) in patients with aspirin-sensitive urticaria. C1 INH deficiency is the basis of hereditary angioneurotic edema. The study was performed on 32 subjects with aspirin-sensitive urticaria. The value of C1 INH in examined patients was the same as in the control group. There seems to be no coexistence of aspirin-sensitive urticaria and C1 esterase inhibitor deficiency.