Objective: Tobacco use has a negative influence on general and oral health but data concerning caries are mainly derived from epidemiological and cross-sectional studies. The aim of this study was to investigate smoking and use of smokeless tobacco (Swedish snus) as determinants of dental caries increment in young adults over 3 years. The baseline cohort consisted of 1295 19-year-olds registered at eight Public Dental Clinics representing socioeconomic strata. After 3 years, 982 of the patients could be reexamined (drop-out rate 24.2%). Caries was scored as decayed and filled surfaces according the WHO criteria and the individual caries increment was recorded by counting the number of surfaces that changed from "sound" to "decayed/filled" over the study period. Information on habitual tobacco use (smoking, snuffing) was collected from a structured questionnaire at baseline. Results: The baseline prevalence of smoking and use of Swedish snus was 22.3% and 6.3% respectively. Smoking, but not snuffing, displayed a statistically significant relationship with caries increment over 3 years. For smoking, the relative risk was 1.5 (95% CI 1.2-1.7) and the number needed to harm 6.8 (95% CI 4.5-14.2). Thus, habitual smoking is a risk factor for caries in young adults and the findings reinforce arguments that dental health professionals should incorporate anti-smoking activities in their preventive strategies.