Two studies have shown that low testosterone is associated with increased mortality.  In a 12 year study men whose total testosterone levels were in the bottom quartile with testosterone less than 240 ng/dl were 40 percent more likely to die than those who were in the highest quartile.  Low testosterone was associated with increased cardiovascular  and  respiratory disease, but not cancer.  Similar results were found in a study of male veterans, where testosterone levels less than 240 were associated with 35% mortality. Normal testosterone levels of greater than 450  showed a mortality rate over five years of 20 percent.

http://archinte.ama-assn.org/cgi/content/abstract/166/15/1660
Low Serum Testosterone and Mortality in Male Veterans Arch Intern Med. 2006;166:1660-1665 Molly M. Shores, MD; Alvin M. Matsumoto, MD; Kevin L. Sloan, MD; Daniel R. Kivlahan, PhD .Conclusions Low testosterone levels were associated with increased mortality in male veterans.
http://jcem.endojournals.org/cgi/content/full/93/1/68
The Journal of Clinical Endocrinology & Metabolism 2008 Jan;93(1):68-75.
Low Serum Testosterone and Mortality in Older Men. Gail A. Laughlin, Eliza2008 Jan;93(1):68-75.beth Barrett-Connor and Jaclyn Bergstrom. Department of Family and Preventive Medicine, School of Medicine, University of California, San Diego, La Jolla, California 92093