Renal cell tumours are characterized by the loss of chromosome 3p and trisomy of 5q segments (common, nonpapillary renal cell carcinoma), or by trisomy of chromosomes 7 and 17 and loss of the Y chromosome (papillary renal cell carcinoma), or by random karyotype changes and mitochondrial DNA alterations (renal oncocytoma). We have studied by means of RFLP analysis the genomic and mitochondrial DNA in 11 chromophobe renal cell carcinomas, which have a unique morphology among kidney cancers. We found a loss of the constitutional heterozygosity at chromosomal regions 3p, 5q, 17p, and 17q, a combination of allclic losses that has not been found in other types of renal cell tumours. Three of the tumours showed a gross alteration in the restriction pattern of the mitochondrial DNA. A combined morphological and genetic analysis suggests that chromophobe renal cell carcinoma is a distinct entity.