Knee extensor resistance training using open kinetic chain (OKC) exercise for patients recovering from anterior cruciate ligament reconstruction (ACLR) surgery has lost favour mainly because of research indicating that OKC exercise causes greater ACL strain than closed kinetic chain (CKC) exercise. In this prospective, randomized clinical trial the effects of these two regimes on knee laxity were compared in the early period after ACLR surgery. Thirty-six patients recovering from ACLR surgery (29 males, 7 females; age mean = 30) were tested at 2 and 6 weeks after ACLR with knee laxity measured using the Knee Signature System arthrometer. Between tests subjects trained using either OKC or CKC resistance of their knee and hip extensors in formal physical therapy sessions three times per week. Following adjustment for site of treatment, pretraining injured knee laxity, and untreated knee laxity at post-training, the use of OKC exercise, when compared to CKC exercise, was found to lead to a 9% increase in looseness with a 95% confidence interval of -8% to +29%. These results indicate that the great concern about the safety of OKC knee extensor training in the early period after ACLR surgery may not be well founded.