We used electrogoniometers to measure the range of movement (ROM) of the knee during various activities, comparing 50 patients with osteoarthritis of the knee (OA) with 20 healthy age- and sex-matched subjects. The minimum and maximum joint angles and the ranges of excursion of the patient and control groups were tested for significant differences, using an unrelated Student’s t-test with pooled variance. Knee flexion in patients with OA was significantly reduced during all activities (p < 0.05), but differences in knee extension were not significant except when patients negotiated stairs. We believe that this reduction in ROM is caused by inhibition due to pain when load-bearing. Static non-load-bearing measurements of the ROM poorly reflected the functional ROM, with a coefficient of determination (r²) of 0.59 in the patient group and 0.60 in the control group. Electrogoniometry of the ROM of the knee provides a reliable, accurate and objective measurement of knee function.