Anterior cruciate ligament (ACL) rupture is among the most common and devastating sports-related knee injuries. 1 Surgical reconstruction is considered the standard of care for patients wishing to return to their previous level play prior to ACL rupture. 2 Over 200,000 ACL reconstructions are performed in the US each year, and the recovery process following surgery can be slow and difficult with persistent strength and endurance deficits. 1 Recovery after ACL reconstruction can take over 1 year, and many patients never return to their pre-injury level of sport participation. 3 It is well known that testosterone levels decrease in response to stress, including the stress of surgery. Testosterone, luteinizing hormone (LH), follicle stimulating hormone (FSH), estrogen, and growth hormone (GH) are among the more commonly studied hormones. 6-9 Even in animal studies, the literature reports contradicting conclusions. 10-11 Some reports have shown increased levels of testosterone to be protective of the ACL, while other studies have shown an increased risk of ACL rupture in patients with higher levels of testosterone.[...] we set out to observe the levels of free testosterone, protein-bound testosterone, LH, and FSH levels in our male patients undergoing ACL reconstruction. The purpose of this study was to explore the response of endogenous hormone production after ACL reconstruction and determine a possible correlation between perioperative testosterone levels in males and postoperative clinical outcomes. Demographic Data and Clinical Outcomes Demographic data, including age, sex, and body mass index (BMI) were obtained through chart review.