Treatment effects and side effects of electroconvulsive therapy (ECT) depend on the level of applied energy in relation to the individual patient's seizure threshold. The threshold is known to increase during a course of frequently repeated ECT treatments and to return to baseline 6 months after cessation of treatment. In continuation ECT, however, as well as after early relapses, the interval between treatments is often longer than a few days but shorter than 6 months. Seizure thresholds for such cases have not been examined. We retrospectively examined 19 patients who had undergone repeated ECT treatments that had been separated by intervals of at least 14 days. We found significant changes in seizure duration, as measured by EEG and the cuff technique, when treatments were separated by durations exceeding 60 days. This is compatible with a decrease in seizure threshold or a loss of anticonvulsant action after 2 months and indicates the necessity to retitrate seizure threshold after this time.