The treatment of epilepsy with carbamazepine (CBZ) may be hampered by cognitive side effects. These side effects are thought to be related to pharmacokinetic properties of the drug. The serum concentration of CBZ is characterized by rapid absorption and a short half-life, which may lead to a considerable fluctuation of the level between doses. Cognitive defects may result from peak levels: short periods with high serum concentration. In a single-blind crossover design, cognitive performance was compared in three conditions. All patients were first tested in the steady state of conventional CBZ. The patients were then assigned randomly to either CBZ-controlled release (CR) or a condition in which conventional CBZ was administered in the same tablet form and dose frequency as CR. Psychological tests were administered four times daily, immediately after the serum samples were taken. A nonmedication control group was tested following the same test scheme to obtain standards for the evaluation of changes in performance during the day. A systematic tendency was found toward higher test performance in the CR condition. This is especially evident for tests of memory and accuracy of visual information processing. The results of the repeated test procedure show that the smoothing effect of the CR condition on serum concentration fluctuations results in a more stable pattern of cognitive functioning during the day.