The aim of this study was to describe the pattern and rate of mineral loss during experimental root surface caries in situ without giving frequent sucrose rinses as a supplementary cariogenic challenge. Six elderly persons carried at total of 18 specimens of unerupted human root surface in recessions of lower partial dentures for 1, 2 and 3 months. Another six root surfaces served as controls. No cleaning of the specimens was allowed during the experimental period whereas the natural dentition was cleaned with a nonfluoride toothpaste. All the specimens, as observed by quantitative microradiography, exhibited subsurface caries lesions. Despite variation between individuals the depth of subsurface demineralization increased linearly with time, the average depth of the lesions being 240 microns after 1 month and 630 microns after 3 months. Delta MC, calculated as the difference in total mineral content between control and test specimens throughout the lesions, increased linearly with time from 2,000 to 8,400 vol% microns after 1 and 3 months, respectively. Preparation for microradiography induced a certain shrinkage of the lesions which resulted in an underestimation of the actual loss of mineral. It is concluded that this experimental model is suitable for studying the etiology and prevention of root surface caries provided the interindividual variation is taken into account.