BACKGROUND The aim of this study was to determine whether graft size is associated with recurrent instability and insufficient functional outcomes after ACL reconstruction. MATERIAL AND METHODS We analyzed 214 consecutive patients with a completed follow-up of 12 months: 55 (25.7%) women and 159 (74.3%) men. Patients were divided into 3 groups according to the diameter of the middle of the hamstring graft. Follow-up examinations were performed pre-surgery and 3, 6, and 12-months postoperatively, and laxity assessments were performed using GNRB®. Differential laxity measured at 134N (Δ134=heathy vs. operated side). A "residual laxity" of the ACLR was defined as Δ134N>3 mm. RESULTS The results of the General Linear Model (Repeated Measures) showed that there was a significant main effect of time factor (F=379.759, p<0.001, η²p=0.681) on differential laxity. We found statistically significant differences (p<0.001) in assessments of differential laxity pre-surgery and at 3, 6, and 12 months postoperatively (time factor). Tegner activity score at 12 months after reconstruction significantly differed when comparing patients with "residual laxity" (4.23±0.83) and others (4.85±1.17) (p=0.038). The study revealed a positive correlation between Tegner activity score at 12 months after surgery and pre-injury Tegner activity score (r=0.728, p<0.001) and negative correlation between age (r=-0.43, p<0.001) or BMI (r=-0.33, p<0.001). CONCLUSIONS Our study revealed that graft diameter is not associated with recurrent instability and does not affect laximetry results. The multiple regression model we developed made it possible to predict the Tegner activity score at 12 months after reconstruction based on pre-injury Tegner activity score, age (years), and BMI (kg/m²) of the patient.