Adult male laboratory mice were exposed for 6 months to a combination of four anabolic-androgenic steroids of the kinds and at the relative levels to which human athletes and body builders expose themselves. The four steroids included testosterone, two 17-alkylated steroids, and an ester, and they were given at doses that totaled either 5 or 20 times normal androgenic maintenance levels for mice. By the time the survivors were 20 months old (1 yr after the termination of steroid exposure), 52% of the mice given the high dose of steroids had died compared with 35% of the mice given the low dose and only 12% of the control mice given no exogenous hormones (P < 0.001). Autopsy of the steroid-treated mice typically revealed tumors in the liver or kidney, other kinds of damage to these two organs, broadly invase lymphosarcomas, or heart damage, and usually more than one of these conditions. It can be concluded that the life span of male mice is decreased dramatically by exposing them for 6 months to the kinds and relative levels of anabolic steroids used by many athletes and body builders.