The ACL-deficient knee has been a management dilemma for many years and, to this day, no refutable plan exists for treatment of this injury. No true prospective study has been performed that evaluates all types of individuals at a variety of activity levels, and, in this day of apparently reliable methods of reconstructing the ACL, it is doubtful that one will occur. The ACL injury is no longer a mystery to the general public; it has received extensive publicity because of injuries of professional athletes and the successful reconstruction in many of these athletes. This article has not completely cleared up the issue of the future of an ACL-deficient knee. It has provided, however, convincing evidence that an active individual with a nonfunctional ACL is susceptible to meniscus injury (R. Barrack, J. Bruckner, J. Kneisl, et al, personal communication, 1990). There is also the risk of more tears occurring with time. Bray and Dandy found in their follow-up of patients with ACL repairs that, if the pivot shift returned, these patients had a much higher incidence of meniscus tears. Many of these studies indicate that, if the meniscus cannot be repaired and requires partial meniscectomy or worse, the articular surface will deteriorate (R. Barrack, J. Bruckner, J. Kneisl, et al, personal communication, 1990). Satku et al showed only 11% incidence of radiographic changes in patients with ACL-deficient knees with no evidence of meniscus tears compared with 100% in those having meniscectomy more than 5 years previously. Activity levels in general also change following this injury. This is probably the most difficult area to assess. Even though a substantial number of persons returned to their preinjury level of activity, it is not always possible to determine if they are playing with the same behavior and attitude. In other words, athletes who are involved in sports with cutting and jumping may modify the need for these activities and yet remain relatively competitive depending on their previous level of skill and the position they play. It has also been shown that many athletes return to their preinjury level initially but with time have significant increase in their symptoms and must modify their level of participation. More individuals limited their activities from the beginning than returned to their preinjury level (R. Barrack, J. Bruckner, J. Kneisl, et al, personal communication, 1990). Instability varies in these individuals and, as in Chick and Jackson's patients, those with mild instability (no rotatory instability) may do reasonably well.(ABSTRACT TRUNCATED AT 400 WORDS)