In web-based health promotion programs, large variations in participant engagement are common. The aim was to investigate determinants of high use of a worksite self-help web-based program for stress management. Two versions of the program were offered to randomly selected departments in IT and media companies. A static version of the program including health screening tool, diary and information about stress was offered to the control group. Additional materials, i.e. interactive, cognitive-based and classical stress management exercises and a chat room, were offered to the intervention group. Baseline data regarding participants' demographics, health (self-ratings and biological measures), lifestyle, work-related factors and group membership were analyzed to study determinants of employees' participation in the program during a period of 12 months. Multiple logistic regression analysis was used and found intervention group membership, being a woman, having at most a secondary education, regular physical exercise habits and having positive expectations of the program were significant predictors of high use. The findings demonstrate that the interactivity of a web-based program is an important factor for determining participation in a web-based worksite stress management program. Implications for those developing and implementing future web-based health promotion activities are discussed. (Contains 4 tables and 1 figure.)