In 92 depressed patients who were randomized to unilateral or bilateral electroconvulsive therapy (ECT) at either low dosage (just above seizure threshold) or high dosage (2.5 times the seizure threshold), subjective side effects were assessed with the Columbia ECT Subjective Side Effects Schedule. A research nurse administered the instrument 4 h after each treatment during the ECT course. In 41 patients, the instrument was also administered before the ECT course. Headache, disorientation, and memory complaints were the most common subjective side effects during the ECT course. Somatic side effects did not change from early to late in the ECT course, and were not influenced by ECT electrode placement or dosage. Most individual somatic side effects, including nausea, tiredness, and muscle aches/pains did not change from pre-ECT to during the ECT course, and may have been a function of the persistent somatic symptoms of depression. Cognitive complaints increased from pre-ECT to during the ECT course, but there was no overall change from pre-ECT to immediately after the ECT course. Cognitive complaints were greater with bilateral compared with unilateral ECT, with no significant effect of electrical dosage. During the ECT course, subjective mood improved and psychomotor agitation decreased, particularly in clinical responders. These findings suggest that most putative somatic side effects are related to the depressive state rather than being induced by ECT. The observed changes reinforce the need to evaluate both subjective and objective side effects during ECT.