A 44-year-old Japanese woman was diagnosed with type 1 hereditary angioedema (HAE) at the age of 30. In March 2007, she began suffering from severe abdominal pain due to intestinal edema. After treatment with C1-INH concentrate, her symptoms disappeared. However, during the subsequent three years, the frequency of the attacks increased continuously, and C1-INH concentrate was necessary for treatment of every attack. The increase in the number of attacks might have been due to the frequent injection of C1-INH concentrate or the deterioration of her disease course. In a genetic investigation, the patient was found to have a novel mutation in the C1-INH gene.