Background: Anterior cruciate ligament (ACL) injury is a severe and common injury, and females have 2-4 times higher injury risk compared to men. Return to sport (RTS) is a common goal after an ACL reconstruction (ACLR), but only about two thirds of patients RTS. Young patients who RTS may have a 30-40 times increased risk of sustaining an additional ACL injury to the ipsi- or contralateral knee compared with an uninjured person. Aims: The overall aim of this thesis was to increase the knowledge about female football players with ACLR, and patients with bilateral ACL injuries, and to identify predictors for additional ipsi- and/or contralateral ACLR. Methods: This thesis comprises four studies. Study I and II were cross-sectional, including females who sustained a primary ACL rupture while playing football and underwent ACLR 6–36 months prior to study inclusion. In study I, 182 females were included at a median of 18 months (IQR 13) after ACLR. All players completed a battery of questionnaires. Ninety-four players (52%) returned to football and were playing at the time of completing the questionnaires, and 88 (48%) had not returned. In study II, 77 of the 94 active female football players (from study I) with an ACLR and 77 kneehealthy female football players were included. A battery of tests was used to assess postural control (the Star excursion balance test) and hop performance (the one-leg hop for distance, the five jump test and the side hop). Movement asymmetries in the lower limbs and trunk were assessed with the drop vertical jump and the tuck jump using two-dimensional analyses. Study III, was a cohort study including all patients with a primary ACLR (n=22,429) registered in the Swedish national ACL register between January 2005 and February 2013. Data extracted from the register to identify predictors for additional ACLR were: patient age at primary ACLR, sex, activity performed at the time of ACL injury, primary injury to the right- or left knee, time between injury and primary ACLR, presence of any concomitant injuries, graft type, Knee injury and Osteoarthritis Outcome Score and Euroqol Index Five Dimensions measured pre-operatively. Study IV was cross-sectional. In this study, patient-reported knee function, quality of life and activity level in 66 patients with bilateral ACL injuries was investigated and outcomes were compared with 182 patients with unilateral ACLR. Results: Factors associated with returning to football in females were; short time between injury and ACLR (0–3 months, OR 5.6; 3–12 months OR 4.7 vs. reference group >12 months) and high motivation (study I). In all functional tests, the reconstructed and uninvolved limbs did not differ, and players with ACLR and controls differed only minimally. Nine to 49% of the players with ACLR and controls had side-to-side differences and movement asymmetries and only one fifth had results that met the recommended guidelines for successful outcome on all the different tests (study II). Main predictors for revision and contralateral ACLR were younger age (fourfold increased rate for <16 vs. >35-year-old patients), having ACLR early after the primary injury (two to threefold increased rate for ACLR within 3 months vs. >12 months), and incurring the primary injury while playing football (study III). Patients with bilateral ACL injuries reported poorer knee function and quality of life compared to those who had undergone unilateral ACLR. They had a high activity level before their first and second ACL injuries but an impaired activity level at follow-up after their second injury (study IV). Conclusions: Female football players who returned to football after an ACLR had high motivation and had undergone ACLR within one year after injury. Players with ACLR had similar functional performance to healthy controls. Movement asymmetries, which in previous studies have been associated with increased risk for primary and secondary ACL injury, occurred to a high degree in both groups. The rate of additional ACLR seemed to be increased in a selected group of young patients who desire to return to strenuous sports like football quickly after primary ACLR. Sustaining a contralateral ACL injury led to impaired knee function and activity level.