BOLD-functional connectivity (BOLD-FC) is decreased in Alzheimer’s disease (AD), and suggested to reflect reduced coherence of neural activity. However, the BOLD signal is based on vascular hemodynamics, therefore, impaired perfusion might also contribute to reduced BOLD-FC. Here, 32 AD patients and 22 controls underwent a simultaneous MR/PET, assessing BOLD-FC by resting-state fMRI, baseline CBF by pASL, and neuronal activity by FDG-uptake. Patients’ BOLD-FC, CBF, and FDG-uptake were reduced within the same precuneal regions. BOLD-FC was positively associated with mean CBF, specifically in patients and independently from FDG-uptake. Data indicate that impaired vascular hemodynamics contribute to reduced BOLD-FC in AD.