Objectives: This paper presents findings for dental caries and fluorosis from an October 1990 follow-up survey in Illinois and compares results with those obtained from two similar school-based examinations conducted in 1980 and 1985 within the same communities. Methods: The seven study sites were grouped into four categories according to the approximate relation of their water fluoride concentration to the recommended optimal fluoride level for the area. Tests for differences in dental caries and dental fluorosis across the four water fluoride groupings and three examinations were conducted. Results: DMFS scores for children who resided in communities with above-optimal water fluoride levels did not change significantly from 1980 through 1990. At the optimal water fluoride concentration, caries scores did not change substantially from 1980 to 1985; however, the mean DMFS score in 1990, 1.9, was significantly lower than caries levels observed during the two previous exams. In the optimally fluoridated area, the proportion of fluorosed tooth surfaces increased significantly from 1980 to 1985, but then declined by 1990 to the level observed in 1980 for both age groups. Conclusions: The results suggest that: (1) in the optimally fluoridated community, the apparent increase in the prevalence of dental fluorosis observed from 1980 to 1985 did not continue from 1985 to 1990; and (2) at above-optimal water fluoride concentrations, dental fluorosis either remained stable or demonstrated no sustained increase over the decade.