Background: The prevalence of enamel defects in the first permanent molars among children has been reported to be high and may be on the increase, particularly in the European countries. Little information is available on the prevalence of enamel defects in the first permanent molars among children in Australia. The aim of this study was to describe the prevalence of enamel defects in the first permanent molars among school children in Western Australia.Methods: Informed consent was obtained from parents of children attending pre-primary schools within an administrative area of the School Dental Service of Western Australia in metropolitan Perth to participate in the study in 2005. Parents completed a questionnaire about the mother's health status during pregnancy, and their child's health status for the first three years. Participating children were examined during 2006 and 2007 by one examiner. Teeth were classified on the type and extent of enamel defects using the modified DDE index, and deciduous tooth caries experience of molars and canines was recorded using the WHO criteria.Results: Parents of 634 children out of the eligible 1135 (56 per cent participation) completed the questionnaire. Five hundred and eleven children with at least one erupted first permanent molar were examined (mean age 7.1 years, SD 0.5). Of the first permanent molars, 42 per cent upper right; 37 per cent upper left; 47 per cent lower left; and 45 per cent lower right were without enamel defects. White diffuse opacities were the predominant enamel defects found with 46 per cent upper right; 47 per cent upper left; 40 per cent lower left and 41 per cent lower right molars affected. Demarcated opacities alone or in association with other defects affected 11 per cent upper right; 14 per cent upper left; 10 per cent lower left and 9 per cent lower right molars. At the individual level, 71 per cent had permanent molars with enamel defects; 47 per cent with white diffuse opacities and 22 per cent with demarcated opacities.Conclusions: Prevalence of enamel defects among this school population is high. Majority of defects were white diffuse opacities and the prevalence of demarcated opacities is at the high end of that reported for some European countries. Possible risk factors for enamel defects remains to be tested.