Intratubular epithelial dysplasia (IED) of the renal tubules has not been fully described in human renal cell carcinoma (RCC). This lesion has been found in male Syrian hamsters exposed to estrogens. One article reports IED in human kidney showing nephrosclerosis and RCC. We examined "normal" kidney tissue adjacent to 110 cases of RCC in an attempt to identify possible precursor lesions. There were 73 male and 37 female patients (M/F = 2:1). The ages ranged from 27 to 86 years (median 64 years). IED was identified in 30 cases. The lesions consisted of foci of crowded tubular epithelium with large, vesicular nuclei two to three times the size of nuclei of benign tubular cells with eosinophilic macronucleoli. The tubules were occasionally filled with dysplastic cells mimicking carcinoma in situ. The lesions were predominantly cortical and periglomerular. They either were subtle and focal or, less commonly, involved tubules diffusely. Eighteen of the 73 male patients (24%) had these lesions compared with 12 of 37 female patients (32%). They were more usually seen in the clear cell (21 of 66) and sarcomatoid (three of four) variants of RCC than in the oncocytic/granular cell (four of 25) or tubulopapillary (two of 14) variants. One case of collecting duct RCC showed no evidence of IED. Immunohistochemical assessment of 20 dysplastic and 20 nondysplastic lesions with their adjacent RCC for cytokeratin, vimentin, cathepsin-D, and epidermal growth factor receptors was inconclusive. Our findings suggest that IED associated with RCC might represent previously unrecognized precursor lesions along the spectrum ranging from dysplasia to frank carcinoma. The biological significance of these lesions, their preponderance in women, and the phenotypic and genotypic characteristics require further investigation.