Hereditary angioedema (HAE) is an autosomal dominant disorder caused by a deficiency of C1 esterase inhibitor (C1-INH). Affected individuals have attacks of swelling involving almost any part of the body. We studied a family with 15 living members, including a 16-year-old girl who had 3 attacks of angioedema in 2 years. Her paternal uncle had died of asphyxiation during an attack 15 years previously. We analyzed the blood of each family member for C3, C4, and C1-INH levels and sequenced the SERPING1 (formerly C1NH) gene that codes for C1-INH. Six individuals had decreased serum levels of C4 and C1-INH, and they were all found to have a single nucleotide A deletion at codon 210 of the gene, 1210fsX210, a novel mutation that accounts for the HAE in this family.