Objectives: Our aim was to investigate the occurrence of untreated caries among Mongolian dental students, compare it between cohorts in two different survey-years and associate it with their preventive knowledge and oral self-care behavior. Methods: A questionnaire assessed preventive knowledge and oral self-care behavior for all clinical-year dental students in 2000 (n=79) and in 2002 (n=73) at the Dental School of Mongolian National Medical University. The dental chart included was for reporting their dental health by tooth. Based on these charts, occurrence of untreated caries was calculated for each cohort. Untreated caries was defined as having one or more decayed teeth (DT>0). Preventive knowledge was assessed by 14 statements on preventive matters. Oral self-care was determined by students’ toothbrushing frequency, use of fluoridated toothpaste, and consumption of sugar-containing food between meals. Statistical significance of differences was evaluated by chi-square test. Logistic regression models were applied to relate students’ having untreated caries to the explanatory variables, one for each survey-year cohort. Results: Occurrence of untreated caries was 15% for the students in 2000 and 34% in 2002. Occurrence was significantly higher among the fourth-year students in 2002 than among their same study-year counterparts in 2000. Students’ study-year was the only significant factor for having untreated caries among students in 2002, caries occurrence among the fifth-year students being lowest. All other variables in the two models remained nonsignificant. Conclusion: Untreated caries among dental students was unexpectedly high, and there factors other than their preventive knowledge and oral-self care seem to be related to the occurrence. Acknowledgements: Grants to BT by CIMO in Finland, the University of Helsinki, and the Finnish Dental Society Apollonia are warmly acknowledged.