Physical inactivity remains a major health problem among older adults in the US despite the known benefits of a regularly active lifestyle on health and quality of life. PURPOSE To examine the long-term effects of a home-based intervention based on the transtheoretical model of behavior change on physical activity in older adults. METHODS Subjects were 954 community-dwelling older adult (age = 75.4 ± 6.6 years) volunteers for the SENIOR (Study of Exercise and Nutrition in Older Rhode Islanders) Project. The subjects were divided into two groups: exercise intervention (E; n = 464) and control (C; n = 490). The 12-month intervention consisted of a manual, stage-based newsletters, individualized expert system reports and counselor calls. Measurements taken at baseline, 12 months and 24 months included stage of change, the Yale Physical Activity Survey (YPAS), and the Up-and-Go. RESULTS At baseline, 55.2% of subjects were in the action or maintenance stage (A/M). Subjects who received E showed significantly more stage progression than C (p <0.05). There was no difference at 24 months in YPAS summary index score or Up-and-Go scores between E and C. YPAS weekly caloric energy expenditure significantly decreased across time in both groups. At 24 months, 39.4% of subject in a pre-action stage at baseline progressed to A/M. Subjects who progressed to A/M were more likely to receive E, and had higher YPAS summary index scores (44.8 ± 1.9 vs 28.9 ± 1.5; p <0.05) and weekly energy expenditure (6795 ± 406 vs 5332 ± 327 kcal; p< 0.05) at 24 months compared to those who did not progress. CONCLUSIONS The disproportionate number of subjects in A/M at baseline may have masked the efficacy of E. E appeared to be more effective with those individuals at risk for a sedentary lifestyle at baseline.