The accumulating data have shown that a single or certain combinations of proto-oncogenes are genetically altered and acquire oncogenic activities in certain tumor types, through a point mutation and/or overexpression. In the present study, Northern blotting analysis was applied to clinical samples of renal cell carcinomas (RCC) to elucidate expression levels of the c-myc, c-fos and Harvey ras genes in this tumor form. In 11 cases of 23 tumors examined (48%) was shown more than 3-fold expression level of the c-myc gene compared with the matching normal RNA, the c-fos gene in 6 cases (26%), and the Harvey ras gene in one case (4%) showing the concomitant overexpression of two other genes. Overexpression of both the c-myc and c-fos genes was detected in 5 cases (22%). Although the distinct correlation between overexpressions of any genes and the clinical background was not induced, it is considered that these aberrations of proto-oncogenes are involved in oncogenesis of a certain subgroup of RCC.