Patients with Obsessional Slowness (OS) exhibit extreme slowness in the execution of some everyday tasks, such as washing and eating. This may be due to time-consuming rituals, checking behaviour and compulsions. On examination some have neurological signs such as a glabellar tap reflex, cogwheel rigidity or abnormal postures. The purpose of this study was to establish a functional explanation for slowness in this patient group. We have studied 6 OS patients using positron emission tomography (PET) with (15)Oxygen to measure regional cerebral oxygen metabolism and [18F]-6-Fluorodopoa (18F-dopa) to assess the integrity of the presynaptic nigrostriatal system. The findings were of focal hypermetabolism in orbital frontal, premotor and midfrontal cortex, whilst dopa uptake into caudate, putamen and medial frontal cortex was normal. The relationship of these findings to the patients' slowness is discussed.