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The incidence of renal cell carcinoma (RCC) is increasing, largely due to the widespread use of cross‐sectional imaging. Most renal tumors are detected incidentally as small, asymptomatic masses. To study their natural history, the authors prospectively followed a series of patients with this type of lesion who were unsuited for or refused surgery.

Twenty‐nine patients with 32 masses that measured < 4 cm in greatest dimension (25 solid masses and 7 complex cystic masses) were studied. The primary outcome was tumor size, which was calculated as volume over time. All patients were followed with serial abdominal imaging, and each mass had at least three follow‐up measurements. The median follow‐up was 27.9 months (range, 5.3–143.0 months).

Overall, the average growth rate did not differ statistically from zero growth (P = 0.09; 95% confidence interval, − 0.005–0.2 …