OBJECTIVE. Because enhancing renal masses are often surgically excised, we assessed interobserver variability in determining enhancement of renal masses. SUBJECTS AND METHODS. Forty-nine complicated cysts or solid lesions were imaged with helical CT. Simple cysts (n = 20) were included, for a total of 69 lesions. Three radiologists obtained three region-of-interest measurements for each lesion in the unenhanced and nephrographic phases. Enhancement was defined as an attenuation increase of 15 H or more, provided that partial volume effects were not the cause. Observer concordance was defined as agreement among all three observers on whether a lesion was enhancing or nonenhancing. Measurement-interpretation agreement was defined as an observer's measuring an attenuation increase of less than 15 H and interpreting the lesion as nonenhancing or measuring an increase of 15 H or greater and interpreting the lesion as enhancing. RESULTS. For the 20 simple cysts, observer concordance was 100% and measurement-interpretation agreement was 100%. For the 49 complicated lesions, observer concordance was 90% and measurement-interpretation agreement was 84% (41/49). Of the five discordant lesions, three were 1.1 or 1.0 cm in size and were interpreted as being of high attenuation on the unenhanced scans, with enhancement, versus a high- attenuation mass with increasing attenuation attributed to volume averaging. The remaining two discordant observations were due to differing interpretations of normally enhancing renal parenchyma versus cyst wall or tumor nodularity. CONCLUSION. For most renal lesions, the observers agreed on whether enhancement was present or absent. Observers were prone to disagree about enhancement for small lesions (1.0-1.5 cm). Occasional disagreements on classification of larger lesions were due to differing interpretations of enhancing normal parenchyma versus abnormal tissue. Discrepancies between an observer's opinion on enhancement and the results of region-of-interest measurements suggest the possibility of disagreement among multiple observers.