Twenty-two patients with primary osteoarthritis (OA) of the knee were studied to determine the effects of OA on laxity of the knee joint. Laxity was measured with a Genucom Knee Analysis System. Ten knees had mild OA (> 50% preservation of joint space). Fifteen knees had moderate OA (some preservation of joint space, but < 50%). Eighteen knees had severe OA (no joint space). A group of 18 knees from 9 healthy (asymptomatic) subjects of ages similar to those of the OA patients were used as controls. Compared to control knees, severe OA knees had less total anteroposterior (AP) translation (12.2 versus 6.6 mm, p < 0.025) and less total tibial rotation (79 versus 59 degrees, p < 0.01). Compared to early OA knees, knees with severe OA had 57% less average total AP translation (15.2 versus 6.6 mm, p < 0.01), 31% less total varus/valgus rotation (15 degrees versus 10.4 degrees, p < 0.016), and 26% less total internal/external tibial rotation (80.1 degrees versus 59 degrees, p < 0.007). These data indicate that osteoarthritic knees tend to have less laxity than normal knees, probably because of a combination of contracture of the ligaments and pressure of osteophytes against ligaments and other capsular structures.