We empirically examined the validity of ego mechanisms of defense as an explanatory concept for psychological health in 307 middle-aged men who were prospectively followed up for 40 years. Assessed on the basis of a two-hour interview when the men were 47 years old, the maturity (health) of the men's defenses correlated highly with independently assessed outcome measures. Evidence is offered that defensive style provides an independent dimension of mental health. First, childhood variables significantly predicted midlife mental health but not midlife maturity of defenses. Second, the bleaker the childhood the stronger the association of maturity of defenses with adult mental health. Third, the Bond Defense Style Questionnaire, administered to 131 of the 307 men six to eight years after the interview, identified the same styles of defense that were identified earlier by clinical assessment.