ABSTRACT. Objective: Low muscle strength is associated with poorer physical function, but limited empirical evidence is available to prove the relationship between muscle mass and physical function. We tested the hypothesis that persons with lower muscle mass or muscle strength have poorer lower-extremity performance (LEP). Design: A cross-sectional, population-based study. Participants: A cohort of 449 men and women aged 65 years and older living in Amsterdam and its surroundings participating in the second examination (1995-1996) of the Longitudinal Aging Study Amsterdam. Measurements: Leg skeletal muscle mass was measured using dual-energy X-ray absorptiometry. Grip strength was used as an indicator of muscle strength. Timed functional performance tests, including walking and repeated chair stands, were used to assess LEP. Results: After adjustment for body height and age, leg muscle mass was positively associated with LEP in men (regression coefficient .178 (95% confidence interval .013-.343), p=.035). In women an inverse association was observed, which became positive after additional adjustment for body mass index (.202 (-.001-.405), p=.052). Grip strength was positively associated with LEP in men and women. After additional adjustment for behavioral, physiological, and psychological variables, the associations between leg muscle mass and LEP disappeared, whereas grip strength remained to be independently associated with LEP in men (.079 (.042-.116), p=.0001), with a tendency in women (.046 (-.009-.101), p=.11). Results were similar when quartiles of leg muscle mass or grip strength were used. Conclusion: These results suggest that low muscle strength, but not low muscle mass, is associated with poor physical function in elderly men and women. However, prospective studies are needed to investigate the association between loss of muscle mass and physical function.