INTRODUCTION: Month-to-month variation in physical activity levels in a cohort of postmenopausal women participating in a single site clinical trial undergoing lifestyle intervention was investigated before and after lifestyle intervention. METHODS: Participants were Caucasian and African American women (mean age = 57.0 ± 3.0 yr) from the Women on the Move through Activity and Nutrition study. Physical activity was measured subjectively by questionnaire (past week and past year) and objectively by pedometer at the baseline and at the 18-month follow-up. RESULTS: At baseline, before intervention, pedometer steps were highest in the summer months (7616 steps per day), lower in the fall (6293 steps per day), lowest in winter (5304 steps per day), and then rebounded in the spring (5850 steps per day). Physical activity estimates from the past-week subjective measure followed the same seasonal pattern. After 18 months, the lifestyle change group significantly increased their pedometer step counts when compared with the health education group (P < 0.0001). At 18 months, pedometer step counts for the health education group appeared to fluctuate from month to month, whereas month-to-month step counts for the lifestyle change group appeared to remain consistent throughout the year. CONCLUSIONS: These results confirm previous reports that suggest physical activity levels fluctuate throughout the year. Lifestyle intervention, which includes a physical activity component, not only increases step counts but appears to reduce some of variation in physical activity levels over the course of a year in postmenopausal women. © 2009 by the American College of Sports Medicine.