Erosion affects both dentitions but has proved difficult to measure. It has been thought to affect children who are less susceptible to caries but previous studies have been confined to countries where caries prevalence is low. The aims of this study were first, to develop and then investigate the validity of an index to measure erosion in primary teeth. Secondly, to determine the prevalence of erosion in primary incisors amongst kindergarten children in Jeddah and its relationship to caries in the same children. Thirdly, to investigate determinants of erosion and caries in the sample. A sample of 41 exfoliated and extracted primary anterior teeth were scored visually and photographically and scores related to appearance on section. Erosion, caries and rampant caries were then measured in a sample of 987 children. The survey included a questionnaire to parents. Scores used in the index through visual inspection and photographs were related consistently to appearance of lost enamel on section. Sensitivity was 0.94 and specificity was 1.0. Kappa values for repeat assessments all exceeded 0.77. Thirty one percent of the children in the survey had evidence of erosion on clinical examination. For 186 this was confined to enamel but for 123 it involved dentine and/or pulp. Amongst the 727 who had readable photographs, 30% had erosion on photographic and 36% on clinical examination. Agreement was seen between the two methods for 93% of the surfaces included. Caries affected 720 (73%) of the children and rampant caries 336 (34%). More children with caries, (excluding rampant caries), had erosion (36%) than children who were caries free (27%). Vitamin C supplements, frequent use of carbonated drinks and the consumption of fruit syrup from a feeding bottle at bed/nap time as a baby were all related to erosion. These drinks were also related to caries but were part of a larger number of significant factors including socio-demographic measures and oral hygiene practices.