Physical functioning late in life has been shown to be affected by genetic factors. Only a few genetic variants have been suggested to be associated with physical functioning, and this only in selected populations (e.g., young healthy males and elite athletes). Declining physical functioning late in life is a major problem in terms of prevalence, morbidity, functional limitations, and quality of life. It is therefore of interest to find a phenotype reflecting physical functioning which has a relatively high heritability and which can be measured in large samples. Hand grip strength is known to be associated with muscular functioning in other muscle groups and with activities of daily living (ADL) functioning, and it predicts incident disability. We studied 1,757 Danish twin pairs aged 45–96 years, and found that this phenotype has a heritability of 52% (95% confidence interval (CI), 48–55%). A powerful design to detect genes associated with a phenotype is obtained using the extreme discordant and concordant sib pairs, of whom 28 and 77 dizygotic twin pairs, respectively, were found in this study. Hence grip strength is a suitable phenotype for identifying genetic variants of importance to mid- and late-life physical functioning. Genet. Epidemiol. 23:110–122, 2002. © 2002 Wiley-Liss, Inc.