AbstractSeveral factors may be responsible for osteoarthritis after anterior cruciate ligament (ACL) reconstruction. The detrimental effect of the surgical technique may explain part of the progression of the pathologic process. In this study, the effect of ACL reconstruction on articular cartilage was investigated by measuring proteoglycan fragments (PF) in synovial fluid collected from patients who had undergone this operation. Synovial samples were obtained from 44 patients with chronic ACL deficiency aged 26 ± 7 years preoperatively, and from some of them, samples were collected for monitoring at 1 month (n = 22), 3 months (n = 17), 6 months (n = 18) and 12s month (n = 18). Synovial fluid taken from 12 contralateral asymptomatic knees of 12 patients (not necessarily opposite knees of ACL‐deficient cases) served as controls. Preoperative values were significantly larger than controls (P < 0.05). PF level reached its maximum value in the 1st month (P < 0.05) and then gradually decreased. It was significantly lower than preoperative values at 6 and 12 months but still greater than controls (P < 0.05). It seems that surgical trauma affects cartilage metabolism for the first 3 months postoperatively. Although reconstruction of the ACL contributes to articular cartilage homeostasis, a complete return to normal values cannot be achieved in 1st postoperative year at least in knees with chronic ACL deficiency. Long‐term monitoring is needed to see whether these findings are early signs of osteoarthritis. Further studies may more clearly demonstrate the effect of the surgical procedure on knees with acute and subacute ACL deficiency.