The aim of this study was to investigate possible modifications to the DDE Index to make it simpler to use and to make the data collected more meaningful and amenable to analyses and interpretation. After the use of the DDE Index in a National Study in Ireland, initial alterations to the Index were tested on a group of children with enamel defects, in Ireland and New Zealand. The DDE Index was then modified to allow for the measurement of demarcated, diffuse, and hypoplastic defects and their severity. With the Modified Index, the prevalence of defects both on index teeth and all permanent tooth surfaces of 8- and 15-year-old children in fluoridated Cork City and non-fluoridated areas of Cork County and Manchester, U.K., was measured. The prevalence of enamel defects on one or more index teeth of children in the three areas ranged from 30 to 42% in 8-year-olds and from 31 to 38% in 15-year-olds. The percentage of children affected as seen by full-mouth examination was somewhat higher, ranging from 38 to 51% for 8-year-olds and 58 to 63% for 15-year-olds. The percentage of index teeth affected (7 to 14%) was generally higher than for all teeth (5 to 9%). Demarcated opacities were the most common defect seen. Diffuse opacities were found to be the discriminating factor between the fluoridated and non-fluoridated areas. In all areas, the vast majority of diffuse opacities extended over less than 1/3 of the surface area of the teeth affected.(ABSTRACT TRUNCATED AT 250 WORDS)