Objective:Breast cancer in young females was usually considered more aggressive and requires aggressive therapy.We investigated whether early detection and improved treatments changed the patts of characteristics,management and outcomes of young breast cancer patients over time.Methods:Females under 40 years of age diagnosed with breast cancer during the periods 1999-2017 and 1999-2015 were identified in the Fudan University Shanghai Cancer Center (FUSCC) and the population-based Surveillance,Epidemiology,and End Results (SEER) registry,respectively.Clinicopathologic characteristics and treatment information were collected.Patients diagnosed before 2013 were followed up.Results:The proportions of young breast cancer patients were 15.0％ and 5.3％ in the FUSCC and SEER cohorts,respectively.In the FUSCC cohort,there was a significant increase in the proportion of ductal carcinoma in situ (DCIS) (from 8.8％ to 16.9％;P＜0.0001) and it remained stable in SEER cohort.The proportion of T1-stage tumors increased dramatically in the FUSCC cohort (from 35.3％ to 41.9％;P=0.008),whereas it decreased in SEER cohort (from 42.4％ to 33.0％;P＜0.0001).The percentage of estrogen receptor (ER)-positive cancers was consistently increased in both the invasive ductal carcinoma (IDC) and DCIS patients in the two cohorts.Breastconserving surgery and immediate implant reconstruction after masteetomy both exhibited increased use over time in the FUSCC cohort.Both the FUSCC and SEER cohorts showed a significantly better prognosis in the recent time period.Conclusions:With the increased early-stage and ER-positive diseases in young patients as well as better systemic tteatment strategies,improved survival has been observed in recent years.There has been a substantial deescalation in surgical therapies in young breast cancer patients.