OBJECTIVE To determine whether delayed computed tomography (CT) can help confirm vascularity in a neoplasm and differentiate it from a high-density cyst when a well-demarcated homogeneous high-attenuating (> 30-HU) renal mass is incidentally discovered during contrast material-enhanced CT. METHODS In 25 patients, 26 well-demarcated, homogeneous high-attenuating renal masses (mean diameter, 2.5 cm; range, 1-4 cm) detected at initial postcontrast CT were further evaluated with delayed CT (mean, 38 minutes; range, 15-240 minutes) performed with identical parameters. On both the initial postcontrast and delayed CT scans, region-of-interest measurements were obtained in renal masses and in the gallbladder or low-density renal cysts as controls. Correlation with surgical or additional imaging findings was used to determine proof of diagnosis. RESULTS Nine of the masses demonstrated no change in attenuation between initial postcontrast and delayed CT, indicating that they represented avascular lesions consistent with high-density cysts. These cases were confirmed with prior or follow-up imaging studies that demonstrated stability. Seventeen masses (nine surgically proved neoplasms and eight neoplasms that demonstrated interval growth at follow-up or previous CT) demonstrated decreased attenuation at delayed CT compared with initial postcontrast CT, which indicates vascularity. CONCLUSIONS Delayed CT of incidentally discovered well-demarcated homogeneous high-attenuating (> 30-HU) renal masses detected at postcontrast CT enables differentiation of high-density cysts from renal neoplasms by demonstrating deenhancement as a proof of vascularity and, hence, neoplasm.