INTRODUCTION: Adrenocortical carcinoma (ACC) is a rare malignancy with poor prognosis. It is vitally important to predict prognosis and restrict unnecessary adjuvant treatments for patients with ACC. This study aims to confirm the prognostic value of Ki-67 and provide a prognostic evaluation on ACC after primary surgery. METHODS: A total of 66 patients satisfied the inclusion criteria and their complete data were collected and reviewed. The correlation between Ki-67 index and clinicopathologic variables was analyzed using chi-square tests and Pearson's or Spearman's test. Survival curves were generated by Kaplan-Meier analysis and compared with the log-rank test. The Cox regression model was performed to estimate hazard ratios for univariate and multivariate analyses. RESULTS: Of the 66 patients, recurrence was observed in 30 patients (45.5%) and 26 patients (39.4%) died of progressive ACC. The evaluated median overall survival (OS) of the entire study population was 16.5 (range 1-104) months and recurrence-free survival (RFS) was 9.0 (range 0-104) months. Increased Ki-67 expression (> 20% and > 3%) was negatively correlated with OS and RFS (chi-square, P = 0.006 and 0.044, respectively). In multivariate analysis, the Ki-67 index with 20% and 3% cutoff as an independent prognostic factor for OS and RFS was validated [hazard ratio (HR) 3.289; 95% CI 1.345-8.042; P = 0.009 and HR 4.471; 95% CI 1.086-18.410; P = 0.038, respectively]. CONCLUSIONS: Ki-67 is a reliable, convenient, and independent prognostic marker for ACC. Additionally, as an indicator with a divergent prognostic role at different cutoff values (20% and 3%), Ki-67 could be used for stratifying patients with a high risk of death or rapid recurrence.