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Objectives: To investigate the correlations between the initial tumor size and size reduction rate in patients treated with targeted agents. To select the patients who can benefit the most from treatment with targeted agents, it will be necessary to find a tumor characteristic that predicts their effectiveness., Methods: The data from 139 metastatic and 16 primary lesions treated with the targeted agents were retrospectively analyzed. They consisted of 86 sunitinib-treated and 69 sorafenib-treated lesions in 54 patients with metastatic renal cell carcinoma who had undergone treatment from April 2008 to July 2010. The relationship between the longest tumor diameter at baseline and the rate of reduction in tumor size was assessed using the Spearmancorrelation test., Results: A linear, moderate to strong association between the initial tumor size and tumor size reduction rate was shown (correlation coefficient -0.441, P < .001). When these tumors were divided into 2 groups at the threshold value (23.95 mm), which was decided by the receiver operating characteristic curve analysis, the smaller tumors demonstrated a significantly greater size reduction than the larger tumors according to the Mann-Whitney U test (P < .001). Both univariate and multivariate linear regression analyses revealed that only the initial tumor size was associated with the rate of reduction in individual tumors (P < .001)., Conclusions: The initial tumor size was a good predictor of the tumor size reduction. This simple observation could be useful for physicians who treat patients with metastatic renal cell carcinoma. In addition, in assessing clinical trials of targeted agents for metastatic renal cell carcinoma using the ResponseEvaluation Criteria in Solid Tumors, perhaps this association should be considered., (Copyright © 2011 Elsevier Inc. All rights reserved.)