From April 1985 to July 1987, 1,408 healthy white men aged 20–60 years in Paris, France, recruited on an occupational basis, underwent a physical examination and measurements of plasma sex hormones in a cross-sectional study. Both total testosterone and estradbl showed a significant stepwise decrease with age (p < 0.001) starting in the earty adult years, while estrone did not vary. These relations of testosterone and estradbl with age remained significant after adjustment for body mass index, subscapular skinfold, and tobacco and alcohol consumptions, and they were not modified by exclusbn of the men who reported chronic disease. Both the mechanism for the early decrease in testosterone and its clinical significance merit further investigation. Am J Epidemiol 1992;135:783–91.