Smoking is one of the risk factor associated with onset, severity and progression of periodontal disease. AIM: The aim of the study was to examine the smoking behaviour and dental health knowledge of high school students in Riyadh and Belfast. MATERIALS AND METHODS: Eight schools from Riyadh and 6 from Belfast were randomly selected by cluster distribution sampling method. Two hundred and ninety students from Riyadh and 144 from Belfast were included giving an overall response rate of 85%. The age range was between 16-17 years. A questionnaire was used to assess demography, smoking habits, dental health knowledge and oral hygiene practices. RESULTS: The results showed that 18% of students were smokers; 24% in Belfast and 15% in Riyadh (x2 (1) = 4.29: P = 0.04). 24% of students in Belfast and 56% in Riyadh smoked at least 1 cigarette per day. 61% of students had bleeding gums although 85% stated that they brushed their teeth at least daily. Bleeding on brushing was common with 53% of Belfast students compared with 65% from Riyadh. Students in Belfast (2.51 +/- 1.15) had significantly higher mean scores for their knowledge about gum health compared with Riyadh students (2.21 +/- 1.44) (t = 2.29: P = 0.02). There was no differences in knowledge about oral health and smoking between the students. However, non-smokers from Belfast and Riyadh (3.32 +/- 1.60) had greater knowledge about oral health and smoking than those who smoked (2.81 +/- 1.45) (t = 2.73: P = 0.007). There was no difference in knowledge about gum health between smokers and non-smokers. CONCLUSIONS/RECOMMENDATIONS: Smoking is more prevalent in Belfast but more cigarettes are smoked in Riyadh. As non-smokers had greater knowledge of the ill-effects of smoking upon their oral health, there is a need to develop location specific interventions to control smoking habits in late adolescence.