High sensitivity for detecting local brain function differences from subsequent PET images acquired at different cerebral stimulation states requires interscan head motion to be minimized. This motion was measured by an optical lever system during scanning (130 min) of 15 subjects in a dual-dose injection study. Despite motion restriction by a face-mask restraint system, rotations in the sagittal and coronal planes (up to 4.1 degrees and 2.4 degrees, respectively) significantly influenced the measured means and variances of local metabolic differences between states. Hence, adjustments for head movement by retrospective, digital slice realignment or, better, real-time corrections are important.