C1 inhibitor (C1-INH) deficiency [hereditary or acquired angio-oedema (HAE or AAE)] is characterized by recurring episodes of subcutaneous or submucosal oedema. Many different mutations in the C1-INH gene have been identified as a cause of HAE. We investigated the molecular basis of the disease in a Japanese woman with sporadic HAE. Direct sequencing of genomic DNA revealed no point mutation in the C1-INH gene. Quantitative real-time PCR showed that the copy number of the C1-INH gene in the patient was half that of a healthy control. Furthermore, we identified a 650-kbp deletion on the chromosome, which included the C1-INH gene. We evaluated the correlation between the patient’s attacks and her coagulation activity. The levels of D-dimer were high during the angio-oedema attacks, and often exceeded the normal range even during remission, thus the level of D-dimer reflected the activity of HAE in this patient.