The authors explored using the ActiGraph accelerometer to differentiate activity levels between participants in a physical activity (PA, n = 54) or "successful aging" (SA) program (n = 52). The relationship between a PA questionnaire for older adults (CHAMPS) and accelerometry variables was also determined. Individualized accelerometry-count thresholds (ThreshIND) measured during a 400-m walk were used to identify "meaningful activity." Participants then wore the ActiGraph for 7 days. Results indicated more activity bouts/day > or =10 min above ThreshIND in the PA group than in the SA group (1.1 +/- 2.0 vs 0.5 +/- 0.8, p = .05) and more activity counts/day above ThreshIND for the PA group (28,101 +/- 27,521) than for the SA group (17,234 +/- 15,620, p = .02). Correlations between activity counts/hr and CHAMPS ranged from .27 to .42, p < .01. The ActiGraph and ThreshIND might be useful for differentiating PA levels in older adults at risk for mobility disability.