Background Asthma has been a global problem,especially in children.We aim to evaluate the contemporary prevalence and influencing factors of asthma among children aged 3-7 years in Shanghai,China.Methods A random sample of preschool children was included in this study.The International Study of Asthma and Aller-gies in Childhood questionnaire was adopted to assess the childhood asthma.Multivariable logistic regression models were used to evaluate the associations between independent variables and childhood asthma.Results Of 6389 preschool children who were invited to take part in this study,6163 (response rate:96.5％) completed the questionnaire and were included in the analysis.The overall prevalence of asthma was 14.6％ which increased more than six folds from 2.1％ in 1990.Being male,younger age,preterm delivery,being born in spring or autumn,being delivered by elective cesarean section without indication,miscarriage,high socioeconomic status,having allergy history,and exposure to passive smoking,latex paint,and dust were potential risk factors for childhood asthma.Spending more time outdoors(＞ 30 min/day),having indoor plants,and cleaning rooms more frequently were potential protective factors.Conclusions The prevalence of childhood asthma in Shanghai has increased dramatically during the past three decades.The findings about risk and protective factors of childhood asthma could be used to develop appropriate strategies to prevent and control childhood asthma in Shanghai and in other similar metropolitan cities.