This paper introduces and applies an operationalization of mental health as a syndrome of symptoms of positive feelings and positive functioning in life. Dimensions and scales of subjective well-being are reviewed and conceived of as mental health symptoms. A diagnosis of the presence of mental health, described as flourishing, and the absence of mental health, characterized as languishing, is applied to data from the 1995 Midlife in the United States study of adults between the ages of 25 and 74 (n = 3,032). Findings revealed that 17.2 percent fit the criteria for flourishing, 56.6 percent were moderately mentally healthy, 12.1 percent of adults fit the criteria for languishing, and 14.1 percent fit the criteria for DSM-III-R major depressive episode (12-month), of which 9.4 percent were not languishing and 4.7 percent were also languishing. The risk of a major depressive episode was two times more likely among languishing than moderately mentally healthy adults, and nearly six times greater among languishing than flourishing adults. Multivariate analyses revealed that languishing and depression were associated with significant psychosocial impairment in terms of perceived emotional health, limitations of activities of daily living, and workdays lost or cutback. Flourishing and moderate mental health were associated with superior profiles of psychosocial functioning. The descriptive epidemiology revealed that males, older adults, more educated individuals, and married adults were more likely to be mentally healthy. Implications for the conception of mental health and the treatment and prevention of mental illness are discussed.