This paper reports on the results of a field experimental design assessing online support groups testing hypotheses derived from the social identification model of deindividuation effects (SIDE; Lea & Spears, 1992) and social information processing theory (SIP; Walther, 1992). Specifically, it is predicted that individuals in an online support group will not distinguish group members from each other initially, but over time, they will begin to differentiate group members from each other. Research questions are posed about the drivers of peoples' perceptions of their group members' credibility and similarity to themselves, and which perceptions are significant predictors of willingness to accept advice provided by a fellow support group member. Kenny's (1994) social relations analysis (SRA) is used to determine how much group variance on ratings of competence, trustworthiness, goodwill, and homophily is attributable to the person making the judgment (judge), the person being judged (target), the unique relationship between a particular judge and a particular target, and error. For visually anonymous groups, the results of these analyses indicate significant judge variance for trustworthiness at time 1 and competence and goodwill at time 2. For visually identifiable groups, results indicate significant judge variance for trustworthiness at times 1 and 2 and competence across all time periods. Linear mixed modeling procedures are used to determine if perceptions of credibility are significant predictors of one's likelihood to take advice given by a fellow support group member. Results indicate that perceived competence is a significant positive predictor advice provided by a fellow group member in visually anonymous groups, and perceived homophily is a significant positive predictor of confidence in graduate school advice provided by a fellow group member in visually identifiable groups. Perceived goodwill is a significant negative predictor of confidence in graduate school advice provided by a fellow group member in both visibility conditions. These findings provide insight into social influence in online health-related support groups.