The aim of the study was to evaluate the coverage of and time used for caries preventive actions by patient and by dentist's characteristics in relation to caries status and orthodontic treatments in patients from birth to 18 in public dental care in Helsinki City Health Department, Finland. Data on caries prevention for the age groups 0- to 18-year-olds in 1999 were analyzed in relation to patient age, caries status, orthodontic care, and dentist's characteristics. Our study comprised a total of 48,040 patients treated by 140 dentists. Main outcome measures were the coverage of and time used for repeated caries prevention performed during visits following the check-up visit. Nine percent of those under age 6 received repeated caries prevention, 29% of the 6- to 15-vears-olds and 24% of the 16- to 18-year-olds. Nineteen percent in the no-caries group (DMFT/dmft = 0) and 36% in the high-caries group (DMFT/dmft > 0 and DT + dt > or = 3) received repeated caries prevention. The average time a dentist used in repeated caries prevention was 3.6 min (s 3.1: median 2.8; range 0.2-15.4 min) for those in the no-caries and 8.1 (s 6.2; median 7.3; range 0.0-31.8 min) for those in the high-caries group. In each category of caries status, the average time used for repeated caries prevention was shorter for those not under orthodontic treatment. In conclusion, the variation in dentists' caries-preventive treatment practices is wide showing both over- and underuse of caries-preventive actions as judged by needs of patients.