Arrested lesions are more resistant to a new cariogenic challenge, but the degree of surface rehardening needed to achieve this is unknown. The aim of this in situ study was to analyze the acid susceptibilityof newly formed and arrested enamel lesions with known arrestment period and surface microhardness. Six individuals wore an oral appliance with human enamel blocks for 3 periods: (1) 21 days of demineralization due to plaque accumulation and cariogenic challenge, 4 blocks/person (nonfluoride dentifrice); (2) 75 days of arrestment, brushing with fluoride dentifrice, 2 blocks/person; (3) 21 days of demineralization, 5 blocks/person: 1 sound block, 2 demineralized blocks and 2 demineralized and arrested blocks (nonfluoride dentifrice). After period 1, all blocks showed a dull whitish surface characteristic of active, noncavitated lesions. After arrestment, the surfaces assumed a shiny and smooth aspect. The Knoop hardness number (KHN, mean ± SD) of the sound blocks was 307.6 ± 15.0. After period 1, microhardness decreased significantly to 162.6 ± 33.5 KHN (p < 0.001). The microhardness of subsequently arrested lesions (279.8 ± 23.1 KHN) was significantly greater than after demineralization, but lower than that of sound enamel. Arrested enamel did not show a decrease in microhardness when subjected to a new cariogenic challenge and after the same cariogenic challenge showed similar microhardness to sound enamel. The results showed that, although noncavitated lesions probably take years to reach microhardness levels like sound enamel, this does not imply that special care, in addition to the ones normally given to sound tooth surfaces, is necessary.