A questionnaire was sent by post to all dentists practising in the General Dental Service and the Community Dental Service in Scotland; 72% of them (1127) completed and returned it. Three sets of descriptions of carious lesions, ordered on the basis of increasing degree of severity, were supplied. One was based on the radiographic appearance of lesions affecting approximal surfaces, the others on the visual/tactile appearance of lesions affecting free smooth surfaces (buccal and lingual) and occlusal surfaces. For each type of surface the dentists were asked which description indicated the minimum point at which a filling ought to be provided (restorative treatment threshold) in 12- and 30-year-old patients. Overall, the majority of dentists said they would intervene earlier in the case of a 12-year-old than in the case of a 30-year-old patient. Many dentists indicated they would fill an approximal lesion whose radiolucency was confined to the enamel (44.2% for a 12-year-old, 20.1% for a 30-year-old). A further substantial proportion gave a response which suggested that the amelodentinal junction would be the critical decision point (25.8% for a 12-year-old, 19.3% for a 30-year-old). Many also felt that a filling ought to be provided prior to visible cavitation for lesions of the occlusal surfaces (55.1% for a 12-year-old, 22.1% for a 30-year-old); fewer advocated the same treatment threshold for lesions affecting buccal/lingual surfaces (24.1% for a 12-year-old, 9.2% for a 30-year-old).(ABSTRACT TRUNCATED AT 250 WORDS)