OBJECTIVE: To evaluate the efficacy at 6-, 12-, and 24-month follow-up of Keep Active Minnesota (KAM), a telephone and mail-based intervention designed to promote physical activity (PA) maintenance among currently active adults age 50 to 70. METHOD: Participants who reported having recently increased their MVPA to a minimum of 2d/wk, 30 min/bout, (N=1049) were recruited in 2004 and 2005 from one large managed care organization in Minnesota, and randomly assigned to either treatment (KAM; N=523), or Usual Care (UC; N=526) with PA assessed using the CHAMPS questionnaire, and expressed as kcal/wk energy expenditure. RESULTS: We find a sustained, significant benefit of the intervention at 6, 12 and 24 months. kcal/wk expenditure in moderate or vigorous activities was higher at 6 (p<.03, Cohen's d(6m)=.16), 12 (p<.04, d(12 m)=.13) and 24 months (p<.01, d(24 m)=.16) for KAM participants, compared to UC participants. CONCLUSIONS: The KAM telephone- and mail-based PA maintenance intervention was effective at maintaining PA in both the short-term (6 months) and longer-term (12 and 24 months) relative to usual care.