Many studies suggest that mutans streptococci (MS), Lactobacillus (LB), and salivary buffering capacity are important risk factors for dental caries. However, target populations for most studies were children. In adult patients, the same risk factors affect the number of fillings or prostheses or secondary caries. It is therefore important to investigate these risk factors as predictors of caries in adults. In the present study, we evaluated the oral conditions of adult subjects at private dental offices using bite-wing radiographs. Detection of salivary LB level using Dentocult LB had a statistically significant correlation with the number of flat surface caries and approximal caries (P < 0.001). Detection of salivary MS level using Dentocult MS and salivary buffering capacity did not predict dental caries. Thus, detection of salivary LB level using Dentocult LB may be a useful tool for detecting approximal and secondary caries.