Background: Sedentary behavior is an independent risk factor for excess body weight and other health problems. There are no published data on sitting time at work, or how this is related to occupation and sector (branch of business). No published study has shown whether extended sitting at work is compensated for by sitting less during leisure time. Methods: This study used data from a continuous cross-sectional survey, from 2000 to 2005 (N=7720). Workers were asked how many minutes they spent sitting during the preceding day, both at work and in their leisure time. To test differences in sitting times among occupational groups and sectors, descriptive analyses and analyses of variance were carried out in 2006. Results: On average, the Dutch working population reported sitting for 7 hours each day, one third of which was at work. Occupational groups and sectors differed significantly in sedentary behavior, mainly involving sitting periods at work. Workers spending long periods sitting at work did not compensate by sitting less during their leisure time. Conclusions: Workers spend a substantial part of their waking and working time seated. Those who sat for long periods at work did not compensate for this lack of activity by adopting less-sedentary behaviors during leisure time. To prevent health problems, the best approach may be to reduce sedentary behavior at work, when traveling to and from work, and during leisure time. © 2007 American Journal of Preventive Medicine.