Seventeen of 59 patients admitted to hospital for treatment of Obsessive-Compulsive Disorder (OCD) were found to have significant slowness, mainly due to difficulty in initiating goal-directed action and suppressing intrusive and perseverative behaviour. In this subgroup subtle neurological abnormalities were found more frequently than in healthy controls and included loss of motor fluency, hesitancy of initiation of limb movements, speech and gait abnormalities, cogwheel rigidity, complex repetitive movements and tics. Difficulties in cognitive set-shifting and complex spatial-and-shifting abilities were found on neuropsychological testing, but no correlation was found between these disturbances and either the degree of obsessionality or the severity of motor dysfunction. These results suggest that patients with obsessional slowness may have a dysfunction in the frontal-basal-ganglia loop system.