In spite of the paucity of efficacy and safety data, the sales of testosterone and other androgenic products have witnessed explosive growth because of increased media attention and public interest. During the summer of 2000, testosterone-related stories were on the cover of Time, Newsweek, New York Times, and Los Angeles Times. The prescription sales of testosterone that had been growing at 25-30% annual rate since 1993, almost doubled in the year 2000, and have cumulatively increased 500% since 1993 (Source: IMS Sales Data, provided by Reed Selby, Marketing Director for ALZA Corporation). The growing testosterone use in older men, without a clear understanding of its benefits or long-term risks, has raised concern among regulatory agencies. The proposed study by providing definitive information on the effects of testosterone replacement on several measures of efficacy and safety in older men would facilitate an analysis of its risk:benefit ratio.