Caries was produced around class V fillings in human teeth in situ. The material consisted of 44 teeth, including 13 controls. The teeth were scheduled for extraction for orthodontic or prosthodontic reasons and the experimental period ranged from 21 to 180 days. Age of the patients ranged from 11 to 62 years. For the production of caries a preformed orthodontic band was attached to the teeth in order to induce accumulation of plaque material. Following extraction, ground sections were prepared and studied mainly by polarized light microscopy and microradiography. The pathogenesis of observed in vivo lesions was found to correspond to that of the in vitro lesions around fillings described by Hals and Nernaes [1971]. The ‘cavity wall lesion’ was seen in 11 out of 28 experimental cases. In experiments with a duration of 100 days or more, the subsurface demineralization of the ‘outer lesion’ was often followed by a secondary surface demineralization. The positively birefringent zone of enamel caries, seen when examined in quinoline, was common and was observed as early as after 21 days. In dentinal tubules ending at the cavity wall, bodies supposed to be ‘caries crystals’ were observed