Few studies have examined the relationship between baseline fitness and exercise adherence in adults with overweight/obesity during a comprehensive behavioral weight loss program (BWLP). Differentiating individuals who adhere to an exercise prescription based on fitness level may improve future intervention approaches. PURPOSE: To examine the association between baseline fitness and exercise adherence during a 26-week supervised exercise program. METHODS: Data from participants enrolled in an 18-month BWLP combining a calorie-restricted diet and supervised exercise were used in this analysis. The 26-week supervised exercise intervention consisted of moderate intensity exercise (65-75% max HR), three supervised sessions/week, progressing from 20 to 60 min/session by week 13. Baseline fitness (VO2 max) was categorized based on published age and sex norms. Logistic regression was used to examine the association between baseline fitness and a) attrition and b) exercise adherence (attended ≥80% of supervised sessions). The association with adherence was assessed over the entire program (weeks 1-26) and during discrete intervals (weeks 1-4, 5-8, 9-14, 15-20, and 21-26). RESULTS: 69 participants (age 41±9.7 kg/m2, BMI 34±3.8 years, 87% female) were enrolled in the BWLP. Participants were classified based on baseline fitness level: 29% very poor (VO2 max 21.8±4.2 mL/kg/min), 48% poor (24.2±3.1 mL/kg/min), 19% fair (26.4±2.4 mL/kg/min), 3% good (32.2±3.8 mL/kg/min), and 1% superior (38.2 mL/kg/min). 78% (n=54) completed the 26-week supervised exercise intervention. Baseline fitness category was not associated with attrition, nor was it associated with adherence over the entire program (weeks 1-26) or at weeks 1-4, 5-8, 9-14, and 15-20. However there was a positive association between baseline fitness category and adherence during weeks 21-26. Participants categorized as poor or above were 9.4 times more likely to attend ≥80% supervised sessions as compared to those categorized as very poor (95%CI: 1.1-78.5; p=0.039). CONCLUSION: Baseline fitness levels in our study population were surprisingly low. Those starting an exercise program with very poor fitness may struggle with adherence over time as the exercise volume reaches higher levels and may require more coaching during this phase of the program.