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Context: Although it is known that serum testosterone (T) concentrations decline with age, the relative contributions of changes in health and lifestyle to that decline have not been adequately assessed., Objective: The objective of this study was to establish the relative importance of aging, health, and lifestyle in contributing to male T decline., Design: A prospective cohort study of health and endocrine functioning in randomly selected men with a baseline visit (T1, 1987-1989) and two follow-up visits (T2, 1995-1997; T3, 2002-2004) was conducted., Setting: An observational study of men residing in greater Boston, Massachusetts, was conducted., Participants: Participants included 1667 men aged 40 to 70 at baseline; follow-up was conducted on 947 (57%) and 584 (35%) at T2 and T3, respectively., Main Outcome Measures: Main outcome measures included total serum T, calculated free T (FT), and SHBG., Results: There were substantial declines in total serum T and FT levels associated with aging alone. However, many health and lifestyle changes were associated with accelerated decline. A 4- to 5-kg/m2 increase in body mass index or loss of spouse was associated with declines in total serum T comparable to that associated with approximately 10 yr of aging. Results were similar for FT, but fewer factors were associated with SHBG after age was taken into account., Conclusions: Both chronological aging and changes in health and lifestyle factors are associated with declines in serum T. Comorbidities and lifestyle influences may be as strongly associated with declining T levels as is aging itself over the short- to midterm. These results suggest the possibility that age-related hormone decline may be decelerated through the management of health and lifestyle factors.