Self-monitoring has been shown to be a crucial part of initial weight loss success in behavioral interventions. However, little is known about the impact of self-monitoring during the period following initial treatment.The current study examined the role of self-monitoring on weight loss during an initial 6-month intervention period (Phase 1) and a 12-month extended care period (Phase 2) in a group of 167 obese women (M±SD: BMI=37.0±5.1kg/m2, age=59.9±6.2years) enrolled in a behavioral weight loss program.Cluster analysis identified three groups of participants with low, moderate, and high rates of weight loss success during Phase 1 and Phase 2. A one-way ANOVA revealed no significant differences in self-monitoring frequency between groups during Phase 1 (p=.645), but significant differences between all three groups during Phase 2 (p=.001). High success participants completed the most self-monitoring records, followed by the moderate group. The low success group completed the least number of records. Furthermore, self-monitoring during Phase 2 significantly mediated the relationship between extended-care session attendance and percent weight change during that time (95% CI [−.004, −.001], p<.001).These results highlight the importance of continuing self-monitoring after the initial phase of treatment to maintain lost weight.