Four renal "oncocytomas" were observed between July 1974 and January 1975 at University Hospital. A review of all renal cell neoplasms previously classified as carcinomas since 1952 was conducted to determine whether this cluster of tumors represented a recent increase in incidence or whether other renal "oncocytomas" had gone unrecognized in the past. The 23-year review of 194 carcinomas yielded an additional nine "oncocytomas." None had appeared prior to 1964, whereas almost half were found in the 1973-1975 interval alone. While it appears that some "oncocytomas" of the kidney have gone unrecognized in the past, it is also evident that their recent increased incidence is epidemiologically significant. Renal "oncocytomas" and renal cell carcinomas are compared with respect to morphology, clinical presentation, and biologic behavior. Evidence derived from this comparison permits the conclusion that these neoplasms originate from proximal tubular epithelium, are benign clinicopathologic entities, and henceforth should be called proximal tubular adenomas with so-called "oncocytic" features.