In a series of cytogenetically analyzed renal tumors of different histological types (chromophobe carcinoma, clear-cell carcinoma, chromophilic carcinoma, and oncocytoma) and normal renal tissue, the length of the telomeric DNA repeats was assessed using a (TTAGGG)3 oligonucleotide probe. A strong correlation was noted between pronounced telomere shortening and the appearance of telomeric associations of chromosomes. These data suggest an etiologic role of the loss of telomeric DNA repeats in the formation of telomeric associations and a possible involvement of this mechanism in the pathogenesis of chromosome aberrations in human tumors.