This study examined the relationship between perceived (stated) levels of function and the measured (observed) performance in squatting and stair climbing activities of 20 patients with chronic low back pain (CLBP) and 20 healthy volunteers. Subjects were asked through an interview to self-assess their ability to comfortably perform stair climbing and squatting as well as other tolerances. The subjects were then asked to perform the activities and their performance levels were recorded. Results revealed that patients' estimate of squatting and stair climbing abilities as well as their demonstrated levels were significantly lower (p < 0.001) than those of the healthy subjects. There was a significant difference between groups in terms of the time required to perform squatting but not stair climbing. Both groups underestimated their physical capabilities. Findings indicate that the use of actual performance measurement combined with self-report of functional abilities is needed when assessing performance levels of healthy subjects as well as patients with CLBP.