Objectives: Because caries activity may be related to dental plaque acidogenicity, a method was developed for chairside evaluation of pH-lowering activity and lactic acid production by dental plaque. Moreover, this study examined the association of these 2 factors with caries experience on oral examination and with caries activity by following caries incidence for 4 years in a group of preschool children. Method and Materials: A dental plaque sample (2.4 µL) was collected from sound buccal surfaces of maxillary primary second molars using a spoon excavator and placed onto the sensor area of a portable pH meter. Sucrose (30 µL, 228 mmol) was mixed with the plaque sample, and pH changes were monitored for 10 minutes. After pH determination, lactic acid concentration in the plaque-sucrose mixture was measured using a portable lactate meter. Results: Caries experience of subjects correlated with minimum pH (at 10 minutes after sucrose addition) (r = -0.53, P .001) and lactic acid production (r = 0.38, P .001). In addition, increments of both primary tooth caries (?dft) and permanent tooth caries (?DFT) for 4 years correlated with minimum pH (r = -0.47, P .005 and r = -0.38, P .05, respectively). Setting cutoff values of pH and lactic acid concentration at 5.0 and 7.0 mmol/L for ?dft allows screening for caries-susceptible subjects (sensitivity = 0.950 and 0.800, specificity = 0.391 and 0.783, respectively). Conclusion: Plaque in caries-susceptible preschool children displays greater pH-lowering activity and lactic acid production. This method can be applied as a chairside screening test for caries activity and susceptibility for preschool children in dental clinics. Keywords: caries experience, caries incidence, chairside evaluation, dental plaque, lactic acid, pH