The question remains whether reduced cerebral blood flow (CBF) leads to brain atrophy or vice versa. We studied the longitudinal relation between CBF and brain volume in a community-dwelling population. In the Rotterdam Study, 3011 participants (mean age 59.6 years (s.d. 8.0)) underwent repeat brain magnetic resonance imaging to quantify brain volume and CBF at two time points. Adjusted linear regression models were used to investigate the bidirectional relation between CBF and brain volume. We found that smaller brain volume at baseline was associated with a steeper decrease in CBF in the whole population (standardized change per s.d. increase of total brain volume (TBV) = 0.296 (95% confidence interval (CI) 0.200; 0.393)). Only in persons aged >= 65 years, a lower CBF at baseline was associated with steeper decline of TBV (standardized change per s.d. increase of CBF = 0.003 (95% Cl -0.004; 0.010) in the whole population and 0.020 (95% CI 0.004; 0.036) in those aged >= 65 years of age). Our results indicate that brain atrophy causes CBF to decrease over time, rather than vice versa. Only in persons aged >65 years of age did we find lower CBF to also relate to brain atrophy.