The proportion of young breast cancer patients in China is significantly higher than in Western countries, and the clinicopathological characteristics and clinical problems faced by patients in China are different from those in Western countries too, so there is an urgent need to conduct some studies for young breast cancer patients in Asia.This study consisted of two breast cancer cohorts in China. The population-based cohort involved breast cancer cases diagnosed in 2000-2017 via cancer registration system, and we describe the epidemiological characteristics of the young breast cancer in China. The hospital-based cohort, patients eligible for enrollment were breast cancer in young women (≤35 years old) from eight centres in different regions of China, diagnosed and treated for the first time in six time periods (i.e., 2000, 2003, 2006, 2009, 2012, and 2015). Patient demographic characteristics and clinical features were compared among the six time periods using a trend test. The Kaplan-Meier method was used to generate survival curves, and the log-rank test was performed to compare OS and DFS. Univariate and multivariate analyses were carried out using Cox proportional hazards regression to estimate hazard ratios (HR) and 95% confidence intervals (CIs).In the population-based cohort, age-specific incidence and age-standardised percentages of breast cancer cases younger than 35 years were increased. The proportion of breast cancer under the age of 35 is increasing more rapidly in rural areas than in urban areas. In the hospital-based cohort, a total of 1308 young breast cancer patients (≤35 years old) were collected. Proportion of patients treated with adjuvant taxane or anthracycline combined with taxane is gradually increasing over the six time periods, and the proportion of patients undergoing breast-conserving surgery is gradually increasing too. Meanwhile, the patients treated with combined ovarian function suppression (OFS) endocrine therapy gradually increased, and the duration of endocrine therapy gradually longer. There is an increasing trend in 5-year disease-free survival (DFS) rate for the total population in the six time periods between 2000 and 2015, but no significant difference in overall survival (OS) rate was observed. Early staging and longer duration of endocrine therapy were factors associated with a favorable prognosis.The incidence of younger breast cancers under 35 years of age has gradually increased and the pattern of patient care has changed significantly over time, which may contribute to the improved prognosis of younger breast cancer patients.Beijing Medical Award Foundation (YXJL-2020-0941-0763), Beijing Hope Run Special Fund of Cancer Foundation of China (LC2021L04), Chinese Academy of Medical Sciences Clinical Translational and Medical Research Fund (2022-I2M-C&T-A-014).© 2023 The Author(s).