The purpose of this study was to determine: (1) the sensitivity to changes over time for the IKDC form, the Lysholm score, and the Cincinnati knee score, (2) the relationship between the IKDC form, the Lysholm score and the Cincinnati knee score, (3) the criterion validity of each graded variable included in the IKDC form, and (4) if a functional knee test should be included as a graded variable and part of the final result of the IKDC form. We included in this prospective study 120 subjects who underwent ACL reconstruction with follow-up times of 3 and 6 months, and 1 and 2 years after surgery. Outcome measurements were the graded variables of the IKDC form (IKDC1-4 and IKDC-final), the Lysholm score, the Cincinnati knee score, a visual analogue scale for patient's satisfaction, knee joint laxity measurement (KT-1000 knee arthrometer), and two functional knee tests (the triple jump and stairs hopple tests). The IKDC1, IKDC2, IKDC-final, and the Lysholm score were not sensitive to changes over time. The Cincinnati knee score was highly sensitive to changes over time and showed significantly improved outcome between each follow-up. IKDC1-4 showed high criterion validity, indicating that the IKDC1-4 is a good means of documenting clinical examination at one follow-up, but not of detecting changes over time. The functional knee tests were significant outcome measurements after ACL reconstruction, and should be reported separately.