Accumulating data suggest that the amount of use, and not simply the duration in situ, influences the wear and survival of total joint replacements. An electronic, digital pedometer was used to record the number of steps taken by 111 non-randomized volunteers who had had at least one total hip or knee replacement. The patients averaged 4988 steps per day, which extrapolates to approximately 0.9 million cycles per year for each joint of the lower extremity. Average activity ranged widely from 395 to 17,718 steps per day, an approximately forty-five-fold difference. The most active patient walked more than 3.5 times the average number of steps per day. Age was significantly associated with activity (p = 0.048), but there was a high degree of variability (standard deviation, 3040 steps per day). Patients who were less than sixty years old walked 30 per cent more on average than those who were sixty years old or more (p = 0.023). Men walked 28 per cent more on average than women (p = 0.037), and men who were less than sixty years old walked 40 per cent more on average than the rest of the patients (p = 0.011). These data indicate that individual differences in the activity of the patient can be a substantial source of variability in rates of polyethylene wear in vivo. The pedometer is an inexpensive investigational tool with many potential applications, including standardizing wear measurements of joint replacements on the basis of gait cycles rather than time. This quantitative approach may provide prognostic information regarding the survival of joint prostheses. Pedometer data may also be useful for quantitative assessment of walking ability in outcome studies.