Objective: To investigate the prevalence of black tooth stain and its associated factors in the primary teeth of preschool children in Shanghai,China.Analyze the saliva pH value and ionic constituents of children with black stain in order to find the causes of black stain.Methods: Clinical examination and questionnaire survey were used to make a cross-sectional survey of randomly selected 1799 preschool children in shanghai,China.The clinical examination included black tooth stain,caries status,oral hygiene and the questionnaire survey included the basic information and questions associated to black tooth stain.One hundred saliva samples(50 with black stain and 50 without)were used to measure pH value and concentration of calcium,phosphorus and iron.Results: The prevalence in the primary teeth of preschool children in Shanghai was 9.9%,and the prevalence of dental caries of children with black stain was significant lower than children without black stain(46.9%compared with 62.7%,P<0.01).Age,type of kindergarden,history of asthma,oral hygiene and mean dmft were associated to black stain.The saliva pH value,concentration of calcium and iron of children with black stain were higher than children without black stain,while phosphorus is lower,but all of them showed no significant difference.