Tumor ploidy and expression of renal differentiation antigens as recognized by a previously described series of monoclonal antibodies (MAbs) (RC2, RC3, RC4, RC38, RC69, RC154 and G250) were assessed in 48 renal-cell carcinomas. A positive association was found between aneuploidy and development of metastases. Aneuploid tumors showed a loss of antigen expression more frequently than diploid tumors. This difference was statistically significant for antigens recognized by RC3 and RC69. Loss of RC154 reactivity was associated with increased metastatic potential. Clinical stage was the most powerful single prognostic variable but ploidy and RC3 reactivity carried additional prognostic information.