Hereditary angioedema(HAE) is an autosomal dominant disease caused by mutation in the SERPING 1 gene leading to the deficiency or functional defect of C1 esterase inhibitor(C1-INH). Symptoms are recurrent episodes of non-pitting edema that involve the subcutaneous layer of skin or the mucosa of the gastrointestinal or respiratory tract. We experienced manifestation of HAE with early onset in a 4-year-old girl, and delayed onset in her mother, 5 years after the diagnosis made in her daughter. Herein, we report a case of HAE in a 4-year-old girl and her mother, who both had the SERPING 1 gene mutation. A 4-year-old girl was admitted to Chungnam National University Hospital in November, 2011 for rash with itching sense that had an appearance of erythema multiform on the whole surface of her body. Since 1 year ago, she had experienced unilateral painless swelling on her arm which recurred approximately 5 to 6 times. On the second day of admission, she developed fever(38.5°C) and severe abdominal pain. Abdominal pain sustained for 1day, after which both her hands and feet started swelling. Her initial physical examination showed a very itchy rash with target sign overlying her face and whole body. Serum total IgE and specific IgE (unicap for cow’s milk, egg white, wheat, soy, peanut, fish and MAST for food) were all negative. Her C3 level was normal but C4(6mg/dl, normal : 17-45mg/dl) and CH50(3.5U/mL, normal : 23-46U/mL) levels were low. C1-INH level was decreased at 10mg/dl(normal: 19.5-34.5mg/dl). In 2017, her C1-INH activity was checked and revealed low levels(25%, (normal: 70-130%) The patient’s mother was diagnosed with breast cancer in September 2015, and started taking tamoxifen. Afterwards, she developed severe abdominal pain that prolonged for 1day, once a month. She also presented with hand swelling without urticaria or pururitis. She visited the allergy clinic at Daejeon St. Mary’s hospital about her symptoms in 2017. Her C3 level was normal and C4(2.2mg/dl, normal: 10-40) and C1-INH activator(25%, normal: 70-130% ) levels were low. Both the patient and her mother underwent genetic testing, revealing a missense mutation(c.1427C＞T; p.P476L) of SERPING 1 gene. We reported a familiar case of HAE, diagnosed at the early age of a 4-year-old girl with a proven mutation in both the patient and her mother.