Seventy severely depressed patients randomly assigned to receive 8 real or sham ECT were further subdivided on the basis of degree of recovery from depression afterwards. In comparison to a non-depressed control group the depressed patients were impaired on a wide range of tests of memory and concentration prior to treatment, but afterwards performance on most of the tests had improved. Real ECT induced impairments of concentration, short-term memory and learning, but significantly facilitated access to remote memories. At 6 months follow-up all differences between real and sham ECT groups had disappeared. On the majority of tests the previously depressed patients now performed at the same level as the control group. There was some evidence that a subgroup of treatment-resistant patients (poor outcome after real ECT) were significantly more likely to complain of memory problems 6 months later.