Recent studies have shown that orthotopic (transurethral) transplantation of human bladder cancer cell lines into nude mice permits tumor growth that accurately reflects their clinical malignant status in the original host. Thus, such a system allows a unique opportunity to analyze the genetic events involved in the conversion of low-grade bladder cancer, the vast majority of which are curable, to the high-grade life-threatening form of the disease. Since 5-10% of transitional cell carcinomas (TCCs) have been shown to contain a mutated HRAS gene, and protein expression levels of all forms of HRAS have been correlated with TCC progression, we chose to study the contribution of the HRAS oncogene in bladder tumor progression. We evaluated the effects of transfection of normal or mutated HRAS genes into a human TCC, called RT-4, that behaves as a superficial noninvasive papillary tumor after transurethral orthotopic inoculation into athymic nude mice. We found that overexpression of either transfected normal or mutated HRAS genes converted RT-4 cells to express an invasive phenotype remarkably similar in nature to the clinical behavior of high-grade bladder carcinomas. These results suggest a role for overexpressed normal or mutated RAS genes in human bladder carcinoma progression, and highlight the importance of using orthotopic inoculation systems for evaluation of the contribution of oncogenes to malignant tumor progression.