There is limited knowledge of knee laxity in the long term after a complete anterior cruciate ligament (ACL) tear treated without ACL reconstruction. The aim of this study was (1) to describe the clinical course of knee laxity after a complete ACL tear over 15 years, and (2) to study the association between knee laxity and meniscal injuries and the development of knee osteoarthritis (OA). We studied 100 consecutive subjects [mean (SD) age 26 (8) years] presenting with acute ACL injury prospectively. The initial treatment in all subjects was knee rehabilitation without reconstructive surgery. The subjects were examined with Lachman's and pivot-shift tests at baseline, 6 weeks, 3 months, 1 year, 3 years and 15 years after the injury. Sagittal knee laxity was also evaluated with the KT-1000 arthrometer at the 15-year follow-up. During follow-up, 22 subjects were ACL reconstructed due to unacceptable knee instability. There was only a mild remaining knee laxity [median Lachman grade and pivot-shift test value of 1 on a 4-grade scale (0–3)] after 15 years in subjects treated without primary ACL reconstruction. Knees with higher anterior sagittal knee laxity 3 months after the injury had a worse long-term outcome with respect to meniscal injuries and knee OA development.