This paper examines the physiologic and epidemiologic evidence for a widely discussed syndrome termed either 'mid-life crisis', 'male menopause', 'male climacteric', or increasingly, 'andropause'. The paper is divided into 2 parts: (1) a review of evidence from physiologic studies conducted over the last decade that examine endocrine function in aging males; (2) a description of the salient features of an ongoing multidisciplinary epidemiologic study (the Massachusetts Male Aging Study) of a sample of approximately 1700 men aged 40-69 yr, randomly sampled from the general population. This study is markedly different in size and content from studies conducted to date. Preliminary findings suggest that age per se may be a relatively unimportant contributor to endocrine variability and that anthropometrics and life style phenomena may be at least as important.