Total hip joint arthroplasty is performed in increasing numbers--about 0.16-0.2% of population per year in industrial countries. In most cases, an implant is a metallic component articulating with a metal, ceramic, or polyethylene liner, used in the hip, knee, and spine. The metal implants release ions in vivo. Toxic concentrations of ions can lead to many adverse physiological effects, including cytotoxicity, genotoxicity, carcinogenicity, and metal sensitivity. Reference levels of ion concentrations in body fluids and tissues determined by many studies are compiled, reviewed, and presented in this article. The concentrations of ions released from different alloys--including cobalt, chromium, nickel, molybdenum titanium, aluminum, and vanadium--are also presented. This article reviews the literature pertaining to clinical data on metal ion concentrations in patients with metal joint prostheses and laboratory data on the physiological effects of the metals.