['Generally small renal masses grow at slow rate; some of them do not exhibit any growth. Does it mean that they are less aggressive? The aim of our study was to compare the growth of enhancing, non-enhancing solid renal masses, angiomyolipomas and complex renal cystic masses < or =4 cm in diameter.', '12 patients with solid enhancing, 20 with solid non-enhancing, 8 with angiomyolipomas and 25 with Bosniak II/IIF/III and IV masses were enrolled and prospectively studied during the surveillance period. The mean diameter of the lesion at presentation was 3.5; 3.1; 3.2 and 3.9 cm, the mean period of observation was 47; 48; 35 and 48 for each group respectively. All masses were detected incidentally. Progression rates (the largest diameter was calculated) were compared among the groups.', 'The range and mean growth rates among all groups after one year of observation were (0.0-0.3; 0.2 cm); (0.0-0.3; 0.2 cm); (0.0-0.4; 0.3 cm); (0.0-0.3; 0.1 cm); (0.0-0.2; 0.1 cm); (0.0-0.0; 0.0 cm) and (-0.6-0.0; -0.2 cm) for 7 groups respectively. Overall 55.4% of masses did not exhibit any growth. We did not monitor any pathological lymph node enlargement, distant metastases in any group of patients. Overall, 85% of masses that exhibited growth during the first year did so at the end of follow up.'] Are growth rates similar in potentially benign and malignant small renal masses detected incidentally?