BACKGROUND A new variant of hereditary angioedema has been reported during the last decade. Three main characteristics distinguish it from classic hereditary angioedema: normal C1 inhibitor activity, predominance in women, and different genetic alterations. OBJECTIVE To assess the symptoms, laboratory findings, and treatment of a population with type III hereditary angioedema from Northwest Spain. METHODS We studied 29 patients (26 female and 3 male) from 13 different families. RESULTS The 26 female patients showed a similar clinical pattern to the classic forms of hereditary angioedema, and 22 of these patients had the estrogen-dependent phenotype. Three patients had a negative family history, and 1 of the parents was confirmed as an asymptomatic carrier in 2 of them. All had functional C1 inhibitor activity within the normal range in periods without high estrogen levels, but during attacks (in female patients) and pregnancy, activity decreased to below 50%. One male patient had normal C1 inhibitor activity during attacks, and he was initially diagnosed as having idiopathic angioedema. The C4 and antigenic C1 inhibitor levels were always normal. All studied patients had the c.1032C>A, Thr309Lys mutation in the factor XII gene. The mutation was also found in asymptomatic relatives: 5 of 6 men studied and 1 of 8 women studied. CONCLUSIONS Positive family history is a diagnosis criterion, but it could be lacking because there may be asymptomatic relatives, primarily males.