Female gender is a risk factor for sustaining anterior cruciate ligament (ACL) injury. However, little is known about possible sex differences in patients with ACL injury/reconstruction.To study sex differences in patient-reported outcomes before and at 1 and 2 years after ACL reconstruction and to present reference values.Cohort study; Level of evidence, 2.Between 2005 and 2008, 10164 patients (mean age, 27 years; SD, 9.8; 42% females) with primary ACL reconstruction were registered in the Swedish national knee ligament register. There were 4438 (44%) of these patients (42% females) who had completed the knee-specific questionnaire, Knee injury and Osteoarthritis Outcome Score (KOOS), and 5255 (52%) who had completed the generic score of health status, EQ-5D, before surgery and were included in this study. Independent t tests were used to study sex differences in KOOS and EQ-5D preoperatively, 1 and 2 years postoperatively, and over time.Preoperatively, female patients reported worse scores than male patients in 4 KOOS subscales (pain, symptoms, sport/recreation, quality of life) and EQ-5D, with the largest difference seen in KOOS sport/recreation (mean difference, 4.7; 95% confidence interval [CI], 3.0-6.3). At 1 year postoperatively, female patients reported worse scores than male patients in KOOS pain (mean difference, 1.4; 95% CI, 0.4-2.4) and KOOS sport/recreation (mean difference, 2.7; 95% CI, 0.9-4.4) and at 2 years postoperatively in KOOS sport/recreation (mean difference, 4.4; 95% CI, 2.1-6.7) and KOOS quality of life (mean difference, 2.4; 95% CI, 0.4-4.4). Female patients reported less improvement from 1 to 2 years postoperatively than male patients in KOOS sport/recreation (mean difference, 3.2; 95% CI, 0.3-6.1). In some age groups, female patients reported a clinically relevant worse KOOS sport/recreation score than male patients (mean difference > or =8).Female patients reported statistically significant worse outcomes than male patients before and at 1 and 2 years after ACL reconstruction. In some age groups, this difference was also clinically relevant. There were no clinically relevant sex differences in improvements over time. We suggest that possible sex differences be analyzed in future studies on evaluation after ACL injury/reconstruction.