The objective was to study whether the ability to mount different step heights depends on knee extension strength, and if this dependence varies according to age or sex among elderly persons. Altogether, 198 independently living 75-year-old women, 98 75-year-old men, 117 80-year-old women and 45 80-year-old men participated in the study. Maximal isometric knee extension strength was measured in a sitting position using a custom-built dynamometer chair. Results were standardized for body mass and height. In the stair-mounting test 10 cm high boxes were combined to form steps with heights of 10 to 50 cm. Older subjects, especially the women, exhibited poorer results in both tests. However, when groups based on maximum step height were analysed separately for strength, three-way analysis of variance revealed no sex effects. Although the number of men in the analysis was quite small, it was concluded that the strength requirements for mounting a certain step height were similar for both sexes. Owing to their sex-related lower average strength, old women may be at greater risk than old men of becoming impaired in certain motor tasks.