Lower leg torsion was measured by computerized tomography (CT-scan) in the knees with osteoarthritis. METHODS Experimental studies; Using the Toshiba CT/TCT-60A, the degree of torsion in five dried tibias were measured in three positions of varus, 0 degrees, 10 degrees and 20 degrees; and in two positions of flexion 0 degrees and 20 degrees. Clinical studies; The degree of tibial torsion was measured by CT-scan in 68 adult patients (85 knees) and compared with the torsion in 13 controls (24 tibias). The correlation between tibial torsion and osteoporosity was investigated by measuring CT-density of the third lumbar vertebra. RESULTS No statistically significant difference was found among the results of measurement obtained in five dried tibias placed in three positions of varus and in two positions of flexion. These results indicate that measurement of torsion using CT-scan is not influenced by the position when varus and flexion deformity are less than 20 degrees. Osteoarthritic knees were divided into five radiographic stages. There were significant differences between the lateral tibial torsions for each stage. The lateral tibial torsion was 23.5 degrees in normal adults, 14.1 degrees in stage II, 11.9 degrees in stage III, 7.5 degrees in stage IV and V, for an average of 11.3 degrees. The rate of decrease in lateral tibial torsion was 59.6% in the proximal tibia, 4.2% in the tibial shaft and 36.2% in the distal tibia. The decreases of lateral tibial torsion were correlated with decrease of CT-density of the third lumbar vertebra. CONCLUSIONS It is apparent from this study that there is a correlation between decreasing lateral tibial torsion with the radiographic stage of osteoarthritis of the knee and general osteoporosity.