Objective The study had 2 objectives: first, to evaluate the success of autologous chondrocyte implantation (ACI) in terms of incidence of surgical re-intervention, including arthroplasty, and investigate predictors of successful treatment outcome. The second objective was to derive a tool predicting a patient's arthroplasty risk following ACI. Design In this Level II, prognostic study, 170 ACI-treated patients (110 males [aged 36.8 9.4 years]; 60 females [aged 38.1 10.2 years]) completed a questionnaire about further surgery on their knee treated with ACI 10.9 +/- 3.5 years previously. Factors commonly assessed preoperatively (age, gender, defect location and number, previous surgery at this site, and the preoperative Lysholm score) were used as independent factors in regression analyses. Results At final follow-up (maximum of 19 years post-ACI), 40 patients (23.5%) had undergone surgical re-intervention following ACI. Twenty-six patients (15.3%) underwent arthroplasty, more commonly females (25%) than males (10%; P = 0.001). Cox regression analyses identified 4 factors associated with re-intervention: age at ACI, multiple operations before ACI, patellar defects, and lower pretreatment Lysholm scores (Nagelkerke's R-2 = 0.20). Six predictive items associated with risk of arthroplasty following ACI (Nagelkerke's R-2 = 0.34) were used to develop the Oswestry Risk of Knee Arthroplasty index with internal cross-validation. Conclusion In a single-center study, we have identified 6 factors (age, gender, location and number of defects, number of previous operations, and Lysholm score before ACI) that appear to influence the likelihood of ACI patients progressing to arthroplasty. We have used this information to propose a formula or tool that could aid treatment decisions and improve patient selection for ACI.