Purpose We investigated whether adding the IIF categorization improved the accuracy of Bosniak renal cyst classification, as evidenced by a low rate of progression in IIF lesions and a high rate of malignancy in category III lesions. Materials and Methods We retrospectively reviewed the records of patients with complex renal cysts categorized as a Bosniak IIF or III. Surveillance imaging and pathological outcomes of category IIF cysts were recorded to determine radiological predictors of progression. Pathological outcomes of category III cysts were recorded to determine the malignancy rate. Results A total of 112 patients met study inclusion criteria, of whom 81 were initially diagnosed with a category IIF cyst and 31 had a Bosniak category III cyst. At a median followup of 15 months 14.8% of Bosniak IIF lesions progressed in complexity with a median time to progression of 11 months (maximum greater than 4 years). There were no differences in tumor or patient characteristics between cysts that progressed and those that remained stable. In the 33 patients with Bosniak III lesions who underwent surgical extirpation the malignancy rate was 81.8%. Most patients had low stage, low grade disease and remained recurrence-free at a median followup of 6 months. Conclusions Adding the IIF category has increased the accuracy and clinical impact of the Bosniak categorization system, as evidenced by a low rate of progression in category IIF cysts and an increased rate of malignancy in surgically treated category III lesions compared to those in historical controls.