OBJECTIVES: To review the natural history and biological potential of small renal masses in order to evaluate whether surveillance is an option for treatment of small renal masses. METHODS: Literature search of MEDLINE and additional references from non-MEDLINE-indexed publications concerning surveillance of small renal masses. RESULTS: Because approximately 26-33% of observed small renal masses do not show radiographic growth, it has been suggested that a brief period of active surveillance may be feasible for selected renal masses, with treatment limited to tumours showing growth. Even though tumour growth might be absent or slow, a proportion of these tumours will express significant malignant behaviour. The biological behaviour of a tumour cannot be unambiguously predicted at present. Surveillance of small renal masses should only be considered in elderly and/or infirm patients with competing health risks, in those with limited life expectancy, and in those for whom surgery is not an option. In all other patients, active surveillance can be considered in the context of a study protocol only. In the majority of the patients, nephron-sparing surgery remains the gold standard treatment. CONCLUSIONS: Surveillance should only be considered as an alternative to surgery for the treatment of small renal masses in selected patients. It should always be combined with close follow-up imaging and should be allowed only when the patient and the urologist accept the calculated risk. Long-term, prospective studies are needed to provide a more accurate assessment of the natural history and metastastic potential of small renal masses.