The prevalence of developmental defects of enamel was assessed in 243 children aged 12-14 yr using the FDI Index. The teeth were not cleaned or dried prior to examination for which fibre optic lighting was used. At least one tooth with defective enamel was seen in 63% of children with a demarcated white opacity present in 44% of children. The enamel was abnormal in 11.7% of teeth, diffuse patchy opacities and demarcated white opacities occurring in 4.4 and 4.2%, respectively. Although defects were found most frequently in the maxillary central incisors, the ranking order of prevalence and the distribution for demarcated and diffuse opacities was quite different. Sex, residence, and the common childhood illnesses did not alter the prevalence of defects which was, however, increased significantly in 22 children with a history of a serious illness or accident (0.01 greater than P greater than 0.001). The prevalence of the diffuse opacities was significantly increased with increased exposure to fluoride either in tablets or in the drinking water (0.01 greater than P greater than 0.001).