OBJECTIVE.: We evaluate the 6-month efficacy of Keep Active Minnesota, a phone- and mail-based physical activity maintenance intervention designed for use with adults age 50 to 70 years who have increased their physical activity within the past year. METHOD.: Participants (N=1049) recruited in 2004 and 2005 from one large managed-care organization in Minnesota were randomly assigned to either treatment (N=523) or usual care (N=526) with physical activity assessed using the Community Healthy Activities Model Program for Seniors questionnaire, and expressed as kcal/week expenditures. RESULTS.: Total physical activity at baseline was similar for treatment and usual care participants (p<0.44) as was moderate/vigorous physical activity (p<0.21). Maintenance of physical activity was higher among treatment participants whose mean 6-month change in total kcal/week energy expenditure was -91, compared to -683 for usual care participants (p<0.002). Mean 6-month change in kcal/week expenditure in moderate or vigorous activities was -49 for treatment participants, compared to -612 for usual care participants (p<0.001). CONCLUSIONS.: This phone- and mail-based physical activity maintenance intervention is efficacious at maintaining physical activity at 6 months.