Anterior cruciate ligament (ACL) rupture is a common sporting injury, often managed surgically with patella-tendon or hamstrings-gracilis autograft. Some people who sustain the injury, request information about their prognosis if they choose to forgo surgery and opt for conservative management. Numerous studies provide data on the prognosis of conservatively managed ACL injuries. These studies have not been systematically reviewed. Thus, the aims of this systematic review are to describe the natural history and clinical course of function and proprioception in the conservatively managed ACL-deficient knee, and to identify prognostic factors. We searched MEDLINE, CINAHL, EMBASE, SportDiscus, PEDro and the Cochrane Central Register of Clinical Trials without language restrictions from the earliest record available up to July 2006. We also searched the Science Citation Index, and iteratively searched bibliographies for prospective studies of outcomes (>6 months follow-up) of conservatively managed complete ACL tears. Six criteria were used to assess the methodological quality of included studies. The main outcome measures were self-reported measures of knee function, activity level, performance in functional tasks and knee proprioception. Fifteen studies of variable methodological quality were included in the review. On average, patients with mixed or isolated ACL-deficient knees reported good knee function (87/100 Lysholm knee scale) at follow-up duration of 12-66 months. On average, functional performance assessed with the hop-for-distance test, was in the normal range. From pre-injury to follow-up there was a reduction in Tegner activity level of 21.3%. According to the methods used in the assessed studies, conservatively managed ACL-deficient knees have a good short- to mid-term prognosis in terms of self-reported knee function and functional performance. However, subjects reduced their activity levels on average by 21% following injury.