Objective: Web-based treatment programs are attractive in primary care because of their ability to reach numerous individuals at low cost. Our aim of this meta-analysis is to systematically review the weight loss or maintenance effect of the Internet component in obesity treatment programs. Methods: MEDLINE and EMBASE literature searches were conducted to identify studies investigating the effect of Web-based individualized advice on lifestyle modification on weight loss. Randomized controlled trials that consisted of a Web-user experimental and non-Web user control group were included. Weight changes in the experimental group in comparison with the control group were pooled with a random-effects model. Results: A total of 23 studies comprising 8697 participants were included. Overall, using the Internet had a modest but significant additional weight-loss effect compared with non-Web user control groups (À0.68 kg, P ¼ 0.03). In comparison with the control group, stratified analysis indicated that using the Internet as an adjunct to obesity care was effective (À1.00 kg, Po0.001), but that using it as a substitute for face-to-face support was unfavorable ( þ 1.27 kg, P ¼ 0.01). An additional effect on weight control was observed when the aim of using the Internet was initial weight loss (À1.01 kg; P ¼ 0.03), but was not observed when the aim was weight maintenance ( þ 0.68 kg; P ¼ 0.26). The relative effect was diminished with longer educational periods (P-trend ¼ 0.04) and was insignificant (À0.20 kg; P ¼ 0.75) in studies with educational periods of 12 months or more. Conclusion: The current meta-analysis indicates that the Internet component in obesity treatment programs has a modest effect on weight control. However, the effect was inconsistent, largely depending on the type of usage of the Internet or the period of its use.