Forty patients with obsessive-compulsive disorder (OCD) were compared to matched healthy controls on neuropsychological tests which are sensitive to frontal lobe dysfunction. On a computerized version of the Tower of London test of planning, the patients were no different from healthy controls in the accuracy of their solutions. However, when they made a mistake, they spent more time than the controls in generating alternative solutions or checking that the next move would be correct. The results suggest that OCD patients have a selective deficit in generating alternative strategies when they make a mistake. In a separate attentional set-shifting task, OCD patients were impaired in a simple discrimination learning task and showed a continuous cumulative increase in the number who failed at each stage of the task, including the crucial extradimensional set shifting stage. This suggests that OCD patients show deficits in both acquiring and maintaining cognitive sets. The cognitive deficits in OCD may be summarized as: (i) being easily distracted by other competing stimuli; (ii) excessive monitoring and checking of the response to ensure a mistake does not occur; and (iii) when a mistake does occur, being more rigid at setting aside the main goal and planning the necessary subgoals. Both studies support the evidence of fronto-striatal dysfunction in OCD and the results are discussed in terms of an impaired Supervisory Attentional System.