This study examined the efficacy of augmenting standard weekly cognitive-behavioral treatment for obesity with a self-monitoring intervention during the high risk holiday season. Fifty-seven participants in a long-term cognitive-behavioral treatment program were randomly assigned to self-monitoring intervention or comparison groups. During 2 holiday weeks (Christmas-New Years), the intervention group's treatment was supplemented with additional phone calls and daily mailings, all focused on self-monitoring. As hypothesized, the intervention group self-monitored more consistently and managed their weight better than the comparison group during the holidays. However, both groups struggled with weight management throughout the holidays. These findings support the critical role of self-monitoring in weight control and demonstrate the benefits of a low-cost intervention for assisting weight controllers during the holidays.