Intervention trials that employ a group-randomized trial design require an adaptation of the usual analytic methods to account for the randomization of intact economic/social groups to study conditions and the positive ICC that is implied by such a design. In the absence of valid estimates of the ICCs for the outcomes of interest, investigators designing trials could only guess at how large a problem they faced and how much they would need to increase sample size to compensate. Aside from this paper, we are aware of only one other publication that provides such estimates, and that study provides estimates for only a handful of outcomes. Our purpose here has been to provide a replication and extension of those findings to a broader array of outcomes. The results presented here suggest that worksite-level ICCs for a variety of smoking and health-related outcomes are generally small and that these ICCs can generally be reduced by adjustment for individual-level characteristics. We have demonstrated how information about these ICCs can be incorporated in sample size calculations to avoid designing "underpowered" studies. Our results should assist investigators in planning studies to evaluate the effectiveness of worksite-based health promotion efforts.