Introduction: We sought to evaluate the pathological results of renal masses in comparison with Bosniak III renal cystic lesions to determine the actual malignancy risk.Methods: A retrospective review of Bosniak III renal lesions identified by computed tomography (CT) or magnetic resonance imaging (MRI) were collected from our patients between August 1, 2013 and December 31, 2015 who underwent surgical excision. TNM stage, histology, Fuhrman grade, and maximum lesion size data was collected. Lesion size relationship with prevalence of malignancy was completed by two-tailed t-test, using the homogeneity hypothesis between malignant and benign groups.Results: Fifteen of 25 (60%) of Bosniak III lesions were determined to be malignant. All malignant lesions were classified as either Fuhrman grade 1 or 2 with no evidence of progression to Bosniak IV. Average size of malignant lesions was smaller than those of benign pathology (3.52±1.99 cm vs. 5.66±2.53 cm; p=0.041). Smaller lesions (size <4 cm) were more likely to be malignant than lesions of a larger size (p=0.047).Conclusions: The malignancy risk of Bosniak III renal lesions was 60% in our study. All Bosniak III lesions were of low Fuhrman grade with no evidence of progression. No patient in this study developed metastatic disease within the three-year followup period. Smaller (<4 cm) Bosniak III cysts were more likely to be malignant and lesion size should be taken into consideration when considering management of complex cysts. Active surveillance may be a reasonable option for Bosniak III cystic lesions, regardless of overall size, based upon their universal low grade and no patient developing metastatic disease.