BACKGROUND Links between physical activity and dementia are based primarily on cross-sectional data or studies with unsatisfactory follow-up. OBJECTIVE We leveraged three decades of follow-up from an established cohort to determine whether physical activity in midlife is associated with late-life cognition and dementia. METHODS The Johns Hopkins Precursors study (n = 646) enrolled participants from 1948-1964 and administered questions about physical activity, from which we calculated metabolic equivalents (MET h/day), and exercise from 1978-present. Cognitive tests were administered in 2008. Dementia was adjudicated through 2011. To characterize associations with midlife physical activity, we used linear regression for cognitive tests and Cox proportional hazards models for dementia onset. Models adjusted for age, sex, smoking, diabetes, and hypertension. RESULTS No physical activity measure from 1978 was associated with late-life cognition or onset of dementia. Both MET h/day (β= 0.007, 95% CI: 0.002, 0.013) and regular exercise (β= 0.357, 95% CI: 0.202, 0.513) in 2006, however, were associated with better cognition in 2008. CONCLUSION Findings from this 30-year cohort study that physical activity measured recently, but not in mid-life, is associated with late-life cognition fits with null findings from randomized trials and other observational studies with extensive follow-up. Cross-sectional findings may be misleading due to reverse causation.