Although "double depression"--major depressive disorder superimposed on an underlying chronic depression--is a frequent phenomenon, the concept has not been well formulated clinically and has rarely been described in the research literature. The authors found that 1) 26% of l0l patients who met the criteria for major depressive disorder had an underlying chronic depressive disorder of at least 2 years' duration, 2) "recovery" rates for patients with superimposed depression differed greatly depending on whether recovery was defined as recovery from the major depressive disorder only or recovery from both disorders, and 3) patients with superimposed depression relapsed at a significantly higher rate immediately after recovery from the acute episode of depression than did patients without superimposed depression.