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BACKGROUND The incidence of breast cancer is increasing in Korea. Breast cancer in young Korean women (< 40 years) is rare, but the rate of young age breast cancer incidence in Korea is higher than that in western countries. This study was aimed to evaluate the tumor characteristics of young age breast cancer patients (< 40 years) among Korean women. METHODS Among the Korean women, who were diagnosed with breast cancer from 2010 to 2015, we identified 10,897 cases of nationally representative sample data. The data was made through 10% systematic sampling of the Korea Central Cancer Registry (KCCR). We conducted a chart review survey to collect the data about tumor size, regional lymph node status, estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) status according to the Collaborative Stage version 2 (CSv2) Data Collection System. We described the number and percentage of the breast cancer stage distribution, tumor grade and intrinsic subtypes by the patient age groups (< 40 years, 40-49 years, 50-59 years, and ≥ 60 years), and evaluated the tumor characteristics in young age breast cancer patients (< 40 years). RESULTS The number of young age breast cancer patients was 1,245 (11.4% of < 40 years vs 35.4% of 40~49 years vs 30.8% of 50~59 years vs 22.4% of ≥ 60 years; P<.001). Young age breast cancer patients were more likely to be diagnosed with larger tumors (T2: 41.6% vs 36.4% vs 36.5% vs 38.4%; T3: 10.1% vs 7.3% vs 6.5% vs 6.2 %; P<.0001), more positive lymph node status (41.2% vs 32.7% vs 35.7% vs 32.5%; P<.0001), and higher tumor grade (grade 3, 26.8% vs 19.4% vs 23.5% vs 22.1%; P<.0001). According to intrinsic subtypes using ER, PR, and HER2, triple negative subtype was found more in young age breast cancer (18.2% vs 11.0% vs 12.2% vs 13.5%; P<.0001). CONCLUSION This study shows that young age breast cancer patients (< 40 years) in Korea have more aggressive tumor including advanced cancer stage at diagnosis, higher tumor grade, and triple negative intrinsic subtype. Therefore, we need to identify high risk group for young age breast cancer (< 40 years) and support their active surveillance. These findings using national cohort provide important information for establishing a national strategy of cancer care to manage young age breast cancer patients. Citation Format: Junyup Kim, So-Youn Jung, Eun-Gyeong Lee, Eun Sook Lee, Han-Sung Kang, See Youn Lee, Jai Hong Han, Heein Jo, Hyun hee Kim, Young-Joo Won, Seri Hong, Jae Jun Lee. Tumor characteristics of young age breast cancer patients using a nationally representative sample of the Korea central cancer registry (KCCR) [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS7-62.