The aim of this study was to investigate whether gender, age, stable personality traits, associated meniscus and/or articular cartilage injuries, treatment (non-operative or reconstructive surgery), additional subsequent trauma to the anterior cruciate ligament (ACL)-injured knee and activity level before injury affect the intermediate outcome after ACL injury. The primary outcome was the Knee injury and Osteoarthritis Outcome Score (KOOS). Fifty-seven patients, 18-50 years old with an acute, unilateral ACL tear were included in the study. At the time of inclusion, the patients completed the Tegner score, regarding activity level before injury, and Swedish universities Scales of Personality (SSP). At follow-up, after 5.6 years they completed the KOOS, the Tegner score and a general questionnaire concerning ACL reconstruction and subsequent knee trauma. The subjects who had sustained additional trauma subsequent to their ACL injury had a significantly worse outcome according to the KOOS than those who had not. Furthermore, low ratings in the embitterment scale in the SSP were correlated to a better outcome in the KOOS. Treatment did not affect the KOOS total score. However, the non-operatively treated subjects had a significantly better outcome according to the knee-related quality-of-life domain in the KOOS.