Few studies have examined the impact of motherhood on successful participation in physical activity (PA) interventions. The current study focused on mothers in the Increasing Motivation for Physical Activity or IMPACT study, which aimed to promote PA in sedentary, low-income, ethnically diverse women (74% Latina). The aim of this study was to determine whether certain maternal variables (e.g., number of children, number and intensity of maternal stressors) influenced successful participation in an 8-week, class-based, PA intervention. PA consisted of accumulating 30 minutes or more of moderate-intensity activities (e.g., walking) five or more days a week. Sixty-eight mothers (average age = 32 years) were assessed at baseline and 10 weeks. Paired comparison t-tests demonstrated a significant increase in PA-related energy expenditure from baseline to 10 weeks (p < 0.05). Furthermore, this increase in PA was significantly associated with a decrease in the number of maternal stressors reported over this time period (p < 0.01). Simultaneous regression analyses indicated that (1) having a higher number of maternal stressors at baseline was associated with lower class attendance (p <or= 0.05) and (2) rating these maternal stressors (e.g., unable to control children's behavior) as being more stressful at baseline was associated with lower levels of PA at 10 weeks (p <or= 0.01), independent of the number of children at home. These results suggest that the number and intensity of perceived maternal stressors may negatively impact attempts to become more physically active. Interventions should address such stressors.