Objective: It is important to find ways to predict response to treatments as this may inform treatment planning. We examined rapid response in obese patients with binge eating disorder (BED) who participated in a randomized placebo-controlled study of orlistat administered with cognitive behavioral therapy delivered by guided self-help (CBTgsh) format. Methods: Fifty patients were randomly assigned to 12-week treatments of either orlistat + CBTgsh or placebo + CBTgsh, and were followed in double-blind fashion for 3 months after treatment discontinuation. Rapid response, defined as 70% or greater reduction in binge eating by the fourth treatment week, was determined by receiver operating characteristic curves, and was then used to predict outcomes. Results: Rapid response characterized 42% of participants, was unrelated to participants' demographic features and most baseline characteristics, and was unrelated to attrition from treatment. Participants with rapid response were more likely to achieve binge eating remission and 5% weight loss. If rapid response occurred, the level of improvement was sustained during the remaining course of treatment and the 3-month period after treatment. Participants without rapid response showed a subsequent pattern of continued improvement. Conclusion: Rapid response demonstrated the same prognostic significance and time course for CBTgsh as previously documented for individual CBT. Among rapid responders, improvements were well sustained, and among non-rapid responders, continuing with CBTgsh (regardless of medication) led to subsequent improvements. (C) 2007 Elsevier Ltd. All rights reserved.