BACKGROUND The objective of this study is to evaluate the clinicopathological features of incidental renal cell carcinoma, compared with non-incidental carcinoma. METHODS Between July 1st, 1984 and June 30, 1994, 87 renal cell carcinoma patients were treated at our hospital; 56 had non-incidental renal cell carcinoma and 31 had incidental carcinoma. The clinicopathological features were examined. RESULTS The incidence of incidental cancer ranges from 0 to 66%, and the incidence has increased in recent years. The median value of maximal tumor size was 4.0cm (1.5 approximately 8.0cm) for incidental cancer, and 8.0cm (3.0 approximately 16cm) for incidental cancer, and 8.0cm (3.0 approximately 16cm) for non-incidental cancer, i.e., the incidental cancer was significantly smaller than the non-incidental one (p < 0.001). The pathological stage of the resected non-incidental renal cell carcinoma (n = 47) was pT1, pT2, pT3 and pT4 in 0, 23, 21 and 3 patients, respectively. For the resected incidental renal cell carcinoma (n = 31) 3, 26, 2 and 0 patients showed pathological stages pT1, pT2, pT3 and pT4, respectively; the pathological stage of incidental renal cell carcinoma was significantly lower than that of non-incidental carcinoma (p < 0.001). Eighteen and 29 resected non-incidental renal cell carcinoma were grades 1 and 2, respectively, whereas 17 and 14 resected incidental renal cell carcinomas were in grades 1 and 2, respectively. Vascular invasion by tumor cells was shown in 31 (66.0%) and 8 (25.8%) patients with non-incidental and incidental renal cell carcinomas, respectively; the incidence of vascular invasion in incidental cancer being significantly lower than in non-incidental cancer (p < 0.001). The performance status and general condition in patients with incidental renal cell carcinoma were superior to those in patients with the non-incidental cancer. The 1, 3 and 5-year survival rate of all 87 renal cell carcinoma patients was 81, 62 and 57%, respectively. These rates for patients with non-incidental renal cell carcinoma were 72, 48 and 41%, respectively, and those for incidental cancer patients were 100%. The survival of patients with incidental renal cell carcinoma was significantly better than that of non-incidental carcinoma patients (p < 0.005). CONCLUSIONS Our results suggest that the detection of incidental renal cell carcinoma will increase, and that the prognosis for renal cell carcinoma will improve. However, even in incidental renal cell carcinoma, careful long-term follow up may be necessary, since some tumors are comparatively large and exhibit vascular invasion.