The role of mitochondrial DNA deletions (dmtDNA) in involutional bone loss seen in elderly men and women has never been examined. The present investigation was carried out to determine the extent of dmtDNA in cortical bone of elderly patients undergoing knee and hip arthroplasties. The majority of earlier studies have employed the polymerase chain reaction (PCR) to detect and quantify dmtDNA in different body tissues. In the present study, Southern blotting was used to screen bone biopsies from 30 patients undergoing orthopaedic surgery (mean age±SD 67.5±9.6 years; range 49–87 years). The blotting of PvuII-digested genomic DNA, carried out using mtDNA probes covering the entire span of mtDNA, revealed high levels of deletions in six subjects (mean age±SD 63.0±10.1 years; range 49–78 years) and moderate to low levels of mutations in another 14 subjects (mean age±SD 64.9±8.9 years; range 53–87 years). The importance of this rather high prevalence of dmtDNA in the bone of the elderly is discussed in terms of possible involvement of increased production of oxygen-derived free radicals and oxidative stress, and its possible role in the accelerated bone loss leading to osteoporosis. Copyright © 2001 John Wiley & Sons, Ltd.