Objectives. This study examined the effect of program format and incentives on participation and cessation in worksite smoking cessation programs. Methods. Twenty-four worksites were randomized to 6 conditions that differed in cessation program format and the use of incentives. Programs were offered for 18 months in each worksite. A total of 2402 cigarette smokers identified at baseline were surveyed 12 and 24 months later to assess participation in programs and cessation. Results. A total of 407 (16.9%) of the smoker cohort registered for programs; on the 12- and 24-month surveys, 15.4% and 19.4% of the cohort, respectively, reported that they had not smoked in the previous 7 days. Registration for programs in incentive sites was almost double that of no-incentive sites (22.4% vs 11.9%), but increased registration did not translate into significantly greater cessation rates. Program type did not affect registration or cessation rates. Conclusions. Although incentives increase rates of registration in worksite smoking cessation programs, they do not appear to increase cessation rates. Phone counseling seems to be at least as effective as group programs for promoting smoking cessation in worksites.