Purpose: To estimate the prevalence of periodontitis in Korean adults and to examine the associations between periodontitis and known risk factors. Methods: Using Korean National Oral Health Survey 2006 data, a total of 4,263 people who had taken oral examination, interviewed by questionnaire, and aged 18 or older were sampled. The prevalence of periodontitis measured by Community Periodontal Index (CPI) was calculated and the differences in prevalence according to known risk factors (age, sex, monthly income, education, residential area, tooth-brushing frequency, regular dental visit, smoking, and diabetes) were examined with chi-square test. Logistic regression analysis was performed to see the effects of each risk factor on the risk of having periodontitis. All statistical approaches were reflected national sampling design using Survey procedures in SAS 9.1. Results: The overall prevalence of periodontitis in Korean adults was 10.3%. There existed statistically significant differences in crude prevalence for periodontitis according to the all risk factors. In logistic regression, older age groups (O.R.:2.94-3.71), people living in rural area (O.R.:1.87), and current smokers (O.R.:1.77) were significantly prone to have periodontitis. People who earned monthly income of more than 2 million Korean won (O.R.:0.64) and brushed their teeth two or more times per day (O.R.:0.60-0.62) had significantly lower risk of having periodontitis. Conclusions: About 10% of Korean adults had periodontitis in 2006. People who were older, living in rural region, in lower income status, smoking, less tooth brushing were more likely to have periodontitis.