Cognition can be affected by electroconvulsive therapy (ECT). Good clinical practice includes neuropsychological assessment, although this is seldom a part of routine clinical practice. It looks like a substantial part of patients fail to complete cognitive assessments. This constitutes a problem in the generalizability of published clinical research on cognitive side effects. Most studies of ECT-related cognitive adverse effects do not discuss this important issue of so-called cognitive test nonparticipants. Recent findings suggest that cognitive test nonparticipants are more severely ill, and probably more vulnerable to cognitive side effects.