Knee injuries are the topic of increasingly sophisticated research because of the importance in professional athletics as well as increasing participation in recrea tional sports. The role of conditioning and fatigue in these injuries remains controversial. Ligaments have high collagen content, and a viscoelastic response to stress would be expected. Because of the postulated relationship between laxity and knee ligament injuries, an experiment was designed using highly motivated athletes to test the hypothesis that exercise to the point of muscular fatigue may cause laxity of the knee and thereby place athletes at risk for ligamentous injury to the knee when fatigued. An exercise protocol was designed to produce mus cle fatigue in the hamstring and quadriceps muscle groups. Knee ligament laxity was tested prior to and subsequent to the exercise protocol. To document muscle fatigue, isokinetic testing of right knee flexion and extension power was used several times during the exercise protocol. A knee arthrometer (KT-1000) was used to quantitatively document ligamentous laxity before and after exercise. The results indicated a sig nificant lengthening in knee joint laxity between preex ercise and postexercise in the left knee as measured at 15 and 20 pounds of passive displacement force (P < 0.05). Maximum manual displacement also demon strated a significant increase in joint laxity (P = 0.02). The right knee, which had undergone isokinetic testing, demonstrated a similar tendency but without a statisti cally significant difference before and after exercise. There was no significant preexercise side to side differ ence, but postexercise measurements demonstrated a left-right difference at 15 pounds, 20 pounds, and max imum manual displacement of statistical significance (P < 0.01). These previously unreported findings document an in vivo increase in the anterior laxity of the knee joint due to exercise. The clinical implications to be drawn from this study are that more accurate clinical examination of the knee may be obtained after a short cool-down period in the knee with suspected ligamentous injury, and, in addition, athletes should be encouraged to perform vigorous warm-up exercises when entering organized sports activities after cool-down periods of greater than 15 minutes.