OBJECTIVE: To determine if treatment format will affect the willingness of women aged 25 ± 34 to participate in a program for primary prevention of weight gain. DESIGN: 102 normal-weight women aged 25 ± 34 were randomized to one of three treatment formats (group meetings, correspondence course, no-treatment control). Acceptability was evaluated by determining the proportion of women participating in their assigned format. Ef®cacy was assessed by determining mean weight changes at posttreatment (10 weeks) and 6-month follow-up, and the proportions of women who remained at baseline weights. RESULTS: Signi®cantly fewer women chose to participate in a group format, compared to the correspondence course and no-treatment control (42%, 84% and 62%, respectively). However, the group format produced the largest shortterm changes in weight ( 7 1.9 AE 1.8 kg, 7 1.1 AE 2.1 kg and 7 0.2 AE 1.3 kg, respectively). CONCLUSIONS: The format of prevention programs may in¯uence the willingness of subjects to participate, as well as treatment outcome. Both format acceptability and ef®cacy should be considered in determining the overall effectiveness of a program.

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