Background: Recent epidemiologic evidence points to the health risks of prolonged sitting, that are independent of physical activity, but few papers have reported the descriptive epidemiology of sitting in population studies with adults. Purpose: This paper reports the prevalence of "high sitting time" and its correlates in an international study in 20 countries. Methods: Representative population samples from 20 countries were collected 2002-2004, and a question was asked on usual weekday hours spent sitting. This question was part of the International Prevalence Study, using the International Physical Activity Questionnaire (IPAQ). The sitting measure has acceptable reliability and validity. Daily sitting time was compared among countries, and by age group, gender, educational attainment, and physical activity. Results: Data were available for 49,493 adults aged 18-65 years from 20 countries. The median reported sitting time was 300 minutes/day, with an interquartile range of 180-480 minutes. Countries reporting the lowest amount of sitting included Portugal, Brazil, and Colombia (medians ≤180 min/day), whereas adults in Taiwan, Norway, Hong Kong, Saudi Arabia, and Japan reported the highest sitting times (medians ≥360 min/day). In adjusted analyses, adults aged 40-65 years were significantly less likely to be in the highest quintile for sitting than adults aged 18-39 years (AOR=0.796), and those with postschool education had higher sitting times compared with those with high school or less education (OR=1.349). Physical activity showed an inverse relationship, with those reporting low activity on the IPAQ three times more likely to be in the highest-sitting quintile compared to those reporting high physical activity. Conclusions: Median sitting time varied widely across countries. Assessing sitting time is an important new area for preventive medicine, in addition to assessing physical activity and sedentary behaviors. Population surveys that monitor lifestyle behaviors should add measures of sitting time to physical activity surveillance. Moreover, the use of objective measures to capture the spectrum of sedentary (sitting) and physical activity behaviors is encouraged, particularly in low- and middle-income countries commencing new surveillance activities.