A new phenomenon, found only in Gilles de la Tourette (GTS) patients, and which we have called ‘mental play’, is described. It was compared with the phenomenon of counting, which occurred in both GTS and obsessive-compulsive patients. In the GTS patients both mental play and counting were best characterised as playful impulsions. In contrast to the GTS patients, the counting of the obsessive-compulsive patients was in line with their obsessive-compulsive behaviour. These findings suggest that repetitive symptoms in GTS patients, even when they share superficial similarities with obsessive-compulsive symptoms, should not be diagnosed automatically as obsessive-compulsive.