We have found that the MIB-1 grade, based on tumor differentiation/histological type, necrosis and Ki-67 (MIB-1) score, is a valid and reproducible prognostic system for adult soft-tissue sarcomas. However, there are limited data available on Ki-67 labeling indices (LI) from adult soft-tissue sarcomas for testing the validity of quantitative image analysis. In this study, the records of 146 adult patients with soft-tissue sarcomas of the extremities and trunk were retrieved, and MIB-1 immunostaining was carried out for the grading. The counted MIB-1 LI values and the scores estimated from microscopic observation were defined as the gold standard. The correlation between MIB-1 LI as assessed by computer-assisted image analysis and by microscopic observation was determined. The image analysis -based MIB-1 LI was highly correlated with the microscopic observation-based MIB-1 LI (r = 0.87, 95% confidence interval (CI) = 0.82-0.92). In addition, agreement between the MIB-1 scores was very high (kappa statistic = 0.83, 95% CI = 0.75-0.91), as was the percentage agreement (89%, 95% CI = 82.8-93.6%) between the results from image analysis and microscopic observation. We conclude that quantitative immunohistochemical evaluation of MIB-1 LI by image analysis enables pathologists to improve interobserver agreement in the assessment of MIB-1 score, and can help to objectively assign the correct histological grade to cases of adult soft-tissue sarcoma, resulting in optimal clinical management.