Clinical and roentgenographic examinations do not provide objective information on the functional performance of a diseased knee joint. In the present study, biomechanical parameters of knee joint function for 95 patients (162 knees) with rheumatoid arthritis and degenerative joint disease were studied in a gait laboratory and compared with those for 29 normal subjects. Gait abnormalities of the patient groups were significant and were related more to the functional status of the knee than to patient age and sex or the specific diagnosis. Knee joint disease produced an inefficient gait that seemed to mitigate joint pain (by reducing poorly tolerated compressive forces on the joint) and to increase proprioceptive feedback (in order to increase dynamic muscular control of the unstable joint). Biomechanical parameters were statistically correlated with some clinical and roentgenographic features to allow standardization of functional assessment of the diseased knee joint.