Nine cases of chromophobe cell renal carcinoma are reported, 2 in pregnant women. One neoplasm progressed and the patient died of disease, and this tumor presented the highest mitotic index and focal positivity to Ki-67 antibody. DNA flow-cytometric study discovered 89% of aneuploidy, with 3 tetraploid cases. We conclude that chromophobe cell renal carcinoma is a relatively indolent but clearly malignant neoplasm. We suggest that the mitotic index could be the best method to predict malignant behavior. The high incidence of aneuploid cell lines may be due to structural anomalies; it is not indicative of malignant behavior.