BACKGROUND: Online weight loss programs are increasingly popular. However, little is known about outcomes and associations with website usage among members of free online weight loss programs. OBJECTIVE: This retrospective cohort study examined the association between website usage and weight loss among members of a free commercial online weight loss program (SparkPeople). METHODS: We conducted a retrospective analysis of a systematic random sample of members who joined the program during February 1 to April 30, 2008, and included follow-up data through May 10, 2010. The main outcome was net weight change based on self-reported weight. Measures of website usage included log-ins, self-monitoring entries (weight, food, exercise), and use of social support tools (discussion forums, friendships). RESULTS: The main sample included 1258 members with at least 2 weight entries. They were 90.7% female, with mean (SD) age 33.6 (11.0) and mean (SD) BMI 31.6 (7.7). Members with at least one forum post lost an additional 1.55 kg (95% CI 0.55 kg to 2.55 kg) relative to those with no forum posts. Having at least 4 log-in days, weight entry days, or food entry days per 30 days was significantly associated with weight loss. In the multiple regression analysis, members with at least 4 weight entry days per 30 days reported 5.09 kg (95% CI 3.29 kg to 6.88 kg) more weight loss per 30 days than those with fewer weight entry days. After controlling for weight entry days, the other website usage variables were not associated with weight change. CONCLUSIONS: Weekly or more frequent self-monitoring of weight is associated with greater weight loss among members of this free online weight loss program.