OBJECTIVE: The overall goal of the study was to understand the accuracy of self-reported weight over a 6-month Web-based obesity program. MATERIALS AND METHODS: As part of a larger study, subjects (n=323; 93% female; 28% African American) were randomized to a 6-month Internet-based behavioral weight loss program with weekly group meetings delivered either: (1) entirely by online synchronous chats or (2) by a combination of online chats plus monthly in-person group sessions. Observed weights were obtained at 0 and 6 months for all participants. Self-reported weights were submitted weekly to the study Web site. Differences in Observed and Reported weights were examined by gender, race, and condition. RESULTS: Observed and Reported weight were significantly correlated at 0 and 6 months (r=0.996 and 0.996, ps <0.001 respectively). However, Reported weight underestimated Observed weight by 0.86 kg (p<0.001) at 6 months. Further, there was a significant weight loss effect (p<0.001) with those losing more weight more accurately estimating their Reported weight at 6 months. Additionally, 6-month Reported weight change differed from Observed weight change (difference=0.72 kg, p<0.001), with weight change using Reported weights estimating a slightly larger weight loss than Observed weights. CONCLUSIONS: In general, the accuracy of self-reported weight is high for individuals participating in an Internet-based weight loss treatment program. Accuracy differed slightly by amount of weight lost and was not improved with periodic in-person assessment. Importantly, weight change by self-report was comparable to observed, suggesting that it is suitable for Web-based obesity treatment.