The aim of this study was to estimate the prevalence of smoking among students in an open university in Thailand and to describe smoking patterns in relation to the personal and social characteristics of the sample. A self-administered questionnaire survey was conducted in 2005 with 87 134 open-university students in Thailand; the respondents aged 15 to 60 years (N = 84 315) are the participants in this study. We found a substantial difference in smoking prevalence by sex, with a much higher proportion of smoking male (20.9%) than female (1.0%) students in all sociodemographic categories. Smoking decreased among men with a higher level of education or income; in contrast, among women, higher incomes were associated with more smoking. Most of the smokers started smoking in high school. The findings provide evidence that future policy making should aim to reduce smoking among the younger population in Thailand, and in particular, they point to the need to preserve low smoking rates among women.