OBJECTIVE To determine the incidence of renal cell carcinoma (RCC) detected incidentally and to compare the survival of these patients with that of patients presenting with suspected RCC. METHODS In a retrospective study, 157 patients surgically treated for RCC from 1979 to 1993 were grouped according to whether the tumour was found incidentally (n = 55) or whether the renal tumour was suspected (n = 102). The groups were compared for tumour grade, stage and size, patient age, sex and survival using univariate and multivariate analyses. RESULTS There were significant differences between the groups in the number of patients with T2 (P < 0.001), T3a (P < 0.05), T3b (P < 0.01), T4 (P < 0.05) and M1 (P < 0.05) stages of disease. There was also a significantly greater proportion of patients with grade 1-2 tumours (P < 0.05) in those diagnosed incidentally. The accumulated overall survival rate was 64% at 5 years, 50% in the group with suspected RCC and 86% in those with tumours discovered incidentally over the same period. There were significant differences (P < 0.001) in the distribution of survival in the two groups. CONCLUSIONS The patients with incidentally discovered RCC had a prognostically more favourable tumour stage, grade and size. Pathological stage and lymph node metastases influenced the difference in survival between the groups. As there was no difference in survival with tumour stage (T1-2), we consider that grade, size and particularly pathological stage determine the better prognosis.