Emotionally unstable character disorder (EUCD) is the diagnosis applied to those patients with chronic maladaptive behavior patterns, such as poor acceptance of reasonable authority, truancy, poor work history, manipulativeness, but with a core psychopathological disturbance of depressive and hypomanic mood swings that last hours to days. Their lability is not usually reactive to environmental or interpersonal events. We examined 21 such patients in a six-week, double-blind random assignment cross-over study comparing lithium carbonate to placebo. Using a global measure of mood swings, lithium carbonate was statistically significantly superior to placebo. This was also true for a week's rating of the mean of the daily range of mood swings. We interpret these findings to mean that EUCD is closely related to affective illness and that lithium carbonate joins chlorpromazine as a valuable, and perhaps a superior, therapy for EUCD.