To investigate the influence of pain severity, knee extensor muscle weakness, obesity, depression, and activity on the association between recent knee pain and limitation of usual and fast paced walking, and ability to rise from a chair. A cross sectional analysis of 769 older women (mean age 77.8, range 65-101) with physical disability, but no severe cognitive impairment. Severity of knee pain in the past month was classified as none, moderate, or severe. Mobility was measured using timed performance tests. The prevalence of recent knee pain was 53% (408/769). One third of the women with pain reported it to be severe. In general, knee pain was only significantly associated with limited mobility if severe. Obesity, activity and, to a lesser extent, depression intensified the effects of pain. Knee extensor weakness did not. Obesity was a distinctive risk factor in that it substantially increased the risk of mobility limitation, but only in women with pain. In women who had severe pain, activity (walking more than three city blocks in the past week) increased the risk of walking disability more than inactivity. Depression had a minor, but statistically significant effect on walking ability, but not the ability to rise from a chair. In older women with recent knee pain, a high pain severity score, obesity, and activity are important factors that increase the risk of mobility limitation.