This study compares cognitive function in new referrals with epilepsy well-controlled on single drug therapy with either phenytoin or carbamazepine with that in an untreated control group. Patients receiving phenytoin performed consistently less well on memory tasks than did those untreated or receiving carbamazepine. Although patients on phenytoin overall showed a trend towards poorer performance on a tracking task, higher blood levels of this drug were correlated with better tracking performance. The correlation between blood levels of carbamazepine and tracking performance was the opposite from that of phenytoin. Blood levels of carbamazepine were negatively correlated with measures of anxiety, depression, and fatigue. These findings have implications for the choice of drug in the management of epilepsy and also for the reported claims of a psychotropic effect of carbamazepine.