OBJECTIVE After renal parenchymal sparing surgery, residual defects may persist on imaging studies at the sites of resection. These "pseudotumors" may lead to confusion as to whether a lesion was removed or has recurred. We describe the imaging appearances and behavior of renal pseudotumors following renal parenchymal sparing surgery. METHODS From 1988 to 1997, 32 patients underwent 46 renal parenchymal sparing surgeries for removal of renal cancers, including von Hippel-Lindau disease in 27, hereditary papillary renal cancer in 2 and sporadic disease in 3. A median of 14 tumors (range 1 to 114) were removed in each operation. Thrombin soaked absorbable gelatin sponge was placed in the tumor bed after resection to aid hemostasis. We reviewed all imaging studies performed after these operations to characterize these lesions and gain further understanding into the prevalence and etiology. RESULTS Of the patients, 9 had a total 13 pseudotumors on initial postoperative imaging studies, including Hippel-Lindau disease in 7, hereditary papillary renal cancer in 1 and sporadic renal cancer in 1. Pseudotumors were observed after 10 of 46 operations (22%). All pseudotumors demonstrated round contours and enhancement after contrast media, simulating a tumor, yet, these lesions resolved in a mean of 13 months, leaving only small cortical scars. CONCLUSIONS Renal pseudotumors are commonly seen after renal parenchymal sparing surgery and should not be confused with residual or recurrent disease. Enhancement is likely due to granulation tissue involving the lattice of absorbable gelatin sponge. These lesions usually resolve within a year but can take longer to do so.