Cytogenetic and flow cytometric DNA analyses were performed on 37 tumor samples from 16 patients with renal cell carcinoma. 36 cultures were chromosomally abnormal and 1 was normal. The most frequent abnormality was trisomy or tetrasomy of chromosome 7 (13 of 16 tumors), suggesting that this abnormality might be a primary change in renal cell carcinoma. Abnormalities of chromosome 3 were seen in 9 of 16 tumors (56%). Eight of 11 tumors (73%) with cytogenetic results from multiple tumor samples showed intratumor cytogenetic heterogeneity. Twelve of the 16 patients had aneuploid tumors, and 4 had homogeneously diploid tumors. DNA index correlated with chromosome number. No association was found between cytogenetic results and clinical stage.