Based on the responses of 1,276 general dental practitioners in Ontario to a mail questionnaire, variations in certain aspects of the dentists' diagnostic, preventive and restorative knowledge and beliefs are examined. Despite recent practice recommendations and guidelines indicating that patient need and service selectivity, rather than traditional, routine prescriptions, should be the guiding principle of practice, many dentists seem to follow tradition in defining optimal intervals for dental examinations and prophylaxes, bitewing radiographs and topical fluoride applications. Although the respondents' knowledge and beliefs were correct and consistent with current evidence in many areas, some large deficiencies were identified. These include: a lack of awareness that professional prophylaxis prior to topical fluoride application is usually unnecessary and that sealant applications to certain teeth are not cost-effective; an over-estimation of the speed at which dental caries progress through the enamel; and, despite good knowledge about the opportunities for the remineralization of enamel lesions with fluorides, a tendency to restore enamel caries. In some instances, practitioner beliefs were shown to influence practice. For example, dentists who knew about the slow progression of approximal surface caries also reported more often that bitewing intervals should be longer, and that the placement of a restoration should be delayed until the caries penetrates the dentin. Similarly, correct knowledge about sealant effectiveness was positively associated with high sealant use. If dentists are to be expected to understand and follow emerging diagnostic, preventive and restorative practice recommendations and guidelines, continuing education is required to correct and update some of their knowledge and beliefs, as displayed in the responses to this questionnaire.