Objective: To evaluate the preventive effect of a “modified fluoride (F) toothpaste technique” on the incidence and progression of buccal/ lingual enamel caries among Saudi adults with high caries prevalence. Method: Adults were randomly assigned to test and control groups. Test group patients were instructed to use the provided F toothpaste twice a day as follows: 1, use 2cm toothpaste; 2, brush for 2min; 3, swish the toothpaste slurry around the teeth for about half a minute before spitting it out; and 4, no post-brushing water rinsing and no eating/drinking for 2hr. The patients in the control group (n=57) were instructed to continue using their regular F toothpaste twice a day without any further instructions. The patients’ compliance in the test group was assessed after 2 years. Results: Of 175 adults, 113 completed the study. The test group (n=56) had a lower enamel mean caries incidence 0.56 (sd 1.04) than the control group (n=57), 1.01 (1.00) (p<0.05), with a preventive fraction of 44%. There was no difference regarding enamel caries progression or arrested caries. The caries incidence in the test group was higher in the subgroup of patients who rinsed with water post-brushing and who only brushed once a day (p<0.05). Conclusion: The “modified fluoride toothpaste technique”, as practised over the 2 years in a Saudi population with high caries prevalence, had a preventive effect on the incidence of buccal/lingual enamel caries but not on the progression. Key words: adults, dental restorations, enamel caries, fluoride toothpaste, Saudi Arabia.