Although overweight and obese individuals are turning to Internet communities for social support for weight loss, there is no validated online instrument for measuring the subjective social support experiences of participants in these communities. The authors' objective was to determine whether an online version of a validated paper questionnaire, the Weight Management Support Inventory, is appropriate for measuring social support among members of Internet weight loss communities. The authors administered the paper and online versions of the questionnaire in random, counterbalanced fashion to 199 members of a large Internet weight loss community. Scores for the paper and online versions were comparable in between-subjects and within-subjects comparisons. Convergent validity is suggested by the finding that participants who posted messages on Internet forums several times per day reported more social support than those who posted less frequently. However, the instrumental (tangible) support items did not load significantly on the instrumental support factor, suggesting that instrumental support is not relevant to the social support exchanged among participants in these communities. The authors conclude that the online, modified Weight Management Support Inventory, without items for instrumental support, is an appropriate instrument for measuring social support for weight loss among members of Internet weight loss communities.