Fluoride release to saliva from three glass ionomer cements, Vitrabond, Ketac-Fil, and ChemFil II, was followed for six weeks in vivo. Colony forming units (CFU) of S. mutans present in saliva before and after placement of glass ionomer restorations were analysed. Thirty-six children received 1-6 glass ionomer cement restorations of one of the three materials. Unstimulated saliva was collected and analysed before insertion of restorations (baseline measurement), immediately after treatment, after 3 weeks and after 6 weeks. The fluoride concentration was measured according to a method slightly modified from Ekstrand (1977) and Duckworth et al. (1987). Baseline concentration of fluoride was between 0.038 and 0.050 ppm. Immediately after placement of GIC restorations, fluoride concentrations increased to 0.8 for ChemFil II and 1.2 ppm for Ketac-Fil and Vitrabond. After 3 weeks, concentrations of released fluoride decreased about 35% for all three materials, and after 6 weeks, concentrations decreased another 30%. However, still after 6 weeks, the fluoride concentrations of unstimulated saliva was 10 times higher than the baseline values. The more restored teeth surfaces the higher was the saliva fluoride concentration found. The prevalence of S. mutants in saliva decreased after placement of the GIC restorations.