In the last 20 years, the wide use of abdominal ultrasound and CT scans to diagnose non renal disease has caused an increase in the number of kidney tumours discovered by chance. Our study aimed to compare the anatomopathological features and clinical evolution of kidney tumours discovered incidentally with those of symptomatic kidney tumours. We retrospectively analyzed 338 patients subjected to radical nephrectomy in our Institute between 1979 and 1999, subdivided into two groups: patients with symptomatic tumours vs those with incidentalomas. The following parameters were taken into account: histological type, pathological stage, Furhman's grade and tumour diameter. Finally, overall survival was compared. Of a total of 338 patients, 160 (47.4%), were symptomatic (group 1) and the other 178 (52.6%) had tumours discovered incidentally (group 2), during US in 144 patients (81%), CT in 32 cases (18%), and urography in 2 cases (1%). No significant differences were found as regards the histological type. When the patients were stratified by stage, a greater incidence of tumours in stage T1 was found in group 2 (42.1% vs 29.4%), while advanced forms were more frequent in group 1 (15.6% vs 9%). There were no differences in histological grade between the two subpopulations. The mean size of incidental tumours was 6.1 cm (range: 1.5-20), vs 7.6 cm of symptomatic tumours (range: 2-25 cm). Analysis of overall survival showed a statistically significant difference between the two groups: there were 37 deaths (23.1%) in group 1 vs 24 deaths (13.5%) in group 2 (p < 0.03). Progression of the disease occurred in 47 (29.3%) subjects in group 1 and 41 (23%) in group 2 (p: ns). This study demonstrated significantly longer survival among patients with kidney tumours discovered incidentally. This should be attributed to the lower stage and smaller size of incidentalomas in comparison with symptomatic tumours.