Hereditary angioedema (HAE) is a genetic disease which may be detected serologically. We present a patient with HAE, in whom we examined the gene defect using the polymerase chain reaction. The patient presented with recurrent episodes of abdominal pain, or non-itchy swellings of the hands, feet, and penis. The serum levels of C1 inhibitor (C1-INH) and C4 were below normal. We determined that a single base change (C-->T) at nucleotide 1482 in the seventh exon was present in the C1-INH gene. This mutation converted the codon for the Gln-339 to a premature translation termination codon TAG. A point mutation in the C1-INH gene can cause type I HAE.