Background: The natural history and growth rate of renal cell carcinoma (RCC) have not yet been determined. The growth rates of primary lesions in incidentally found RCC were compared with those of metastatic lesions.

Methods: Sixteen patients who did not receive immediate surgical treatment for renal solid masses that were later proven to be RCC were reviewed retrospectively. All primary lesions of the 16 patients were found incidentally. For comparison, metastatic lesions were evaluated in another 16 patients with RCC. Of these, 11 underwent surgical treatment for the primary lesions.

Results: The growth rates of primary and metastatic lesions of RCC varied. They ranged from 0.10 to 1.35 cm/year for primary lesions and from 0.08 to 7.87 cm/year for metastatic lesions. The growth rate of primary lesions of incidentally found RCC was lower than that of metastatic lesions (P = 0.0159). The initial tumor diameter and pathological grade did not affect the growth rate of the primary lesion of incidentally found RCC. However, a close correlation was found between the growth rate of metastatic lesions and the pathological grade of the primary lesion in patients with metastasis.

Conclusions: The growth rate of incidentally found RCC varied. Some patients with the disease may be candidates for ‘watchful waiting’ when an immediate surgical treatment is not indicated, but they should be selected with great caution.