Background: Interventions that teach strategies for integrating physical activity into a person’s daily routine are becoming more common. These interventions have been found to increase physical activity behavior, although the increases have not been large. The small to moderate changes in physical activity can result from participants having insufficient adherence to the intervention protocol to produce an intervention effect. Given that adherence is likely to affect the power to find a treatment effect, it should be tracked. This study examined changes in adherence over 6 months for a lifestyle physical activity intervention. Methods: Participants were 244 sedentary adults who took part in the Project PRIME lifestyle physical activity intervention. Adherence was assessed separately for a group-based intervention (PRIME G) and a telephone- and mail-based intervention (PRIME C). Markers of adherence were completion of homework, self-monitoring of physical activity, attendance at class (PRIME G only), and completion of monthly telephone calls (PRIME C only). Changes over time in adherence markers and differences between intervention groups for homework completion and adherence to self-monitoring were modeled with generalized estimating equations (GEE). Results: The probability of attending class, completing the telephone calls, and completing the homework decreased significantly over 6 months. Participants only self-monitored an average of 5 to 6 days each calendar month. Participants in the group-based intervention were more likely than those in the telephone- and mail-delivered intervention to complete the homework throughout the study. Conclusions: The findings suggest that individuals are willing to adhere with a telephone call protocol over 6 months. They are less willing to complete homework and attend class over this same time period. Most are not willing to self-monitor their lifestyle physical activities more than a few days a month.