Objective:To evaluate a culturally appropriate intervention to increase activity in overweight Mexican American women. Methods: Participants were randomly assigned to a physical activity program or wait-list control. Results: Treated participants were not more active than controls at 6 or 12 months. In addition, we found no significant differences in the pro-portion of individuals who met an objective criterion for physical activity from baseline to 6 months in the treatment or control groups. Conclusion: The intervention did not increase physical activity in this population. Differences in baseline activity and contamination of the control group may partially account for the outcome. Am J Health Behav 2001;25(4):396-406S edentary lifestyle is a significant problem in the United States, particularly for Mexican Americans. Ac-cording to data from NHANES III, Mexican Americans were the least physically active group, with prevalence rates of little or no leisure time physical activity reaching 65% and 74% for men and women, respectively. 1 As with other populations, the sedentary lifestyle adopted by Mexican Americans is associated with significant health problems including obesity, type 2 diabetes mellitus, and cardiovascular disease. [2][3][4][5][6][7][8] Several studies with other ethnic groups have shown that physical activity, along with modest weight loss, can have a beneficial effect on cardiovascular disease and diabetes risk factors. 9-12 It is likely that Mexican Americans also would experience the same beneficial effects on cardiovascular-, diabetes-, and obesity-related health risks from improvements in physical activity. [13][14][15] Unfortunately, few studies have attempted to develop lifestyle modification programs that focus on physical activity