Second-generation metal-on-metal (M/M) total hip replacements were introduced into clinical use in the late 1980s and demonstrate equivalent survivorship to conventional metal-on-polyethylene prostheses. Wear rates are comparable to those of first-generation designs that survived for a long time in the body. Biological effects from metal ions remain a concern. Patients with both first- and second-generation M/M hips have higher levels of cobalt and chromium in their blood and urine than either patients with metal-on-polyethylene devices or unoperated patients. Concerns include the potential for acquired hypersensitivity, mutagenicity, and carcinogenicity. However, reports of proven adverse effects are scant. Prospective, randomized trials with follow-up in excess of 15 years will be needed to differentiate between the performance and effects of M/M and other bearing combinations. Key words: totalhip arthroplasty, metal-on-metal, wear, biological effects, clinical results.