In order to investigate the relationship between genetic instability and DNA aneuploidy of malignant cells in human solid tumors, we studied the variations of DNA index within a single tumor. Multiple sampling (mean of 6.4 samples per tumor) was performed in 24 renal cell carcinomas (RCC). Based on the variations in DNA indices within a single tumor, RCC were divided into three groups: 1) tumors with stable DNA indices (variation within the range of the measurement error), including all DNA diploid tumors (n = 8), all hyperdiploid tumors (n = 3), a hypodiploid tumor (n = 1), and only 2 of 11 tetraploid and hypotetraploid tumors; 2) tumors with moderate variations in DNA indices, which were all close to tetraploidy (n = 5); and 3) tumors with large variations of DNA indices. In this last group, subclones present within a tumor varied widely in their DNA indices from the tetraploid to the triploid region, reflecting the DNA content instability within the tumor cell population. These results suggest that DNA aneuploidy can arise by two different mechanisms: 1) loss or gain of chromosomes leading to hypodiploid or hyperdiploid tumors with no apparent increase in DNA content instability, and 2) doubling of the chromosome set followed by random loss of chromosomes as suggested by the DNA indices ranging from tetraploid to triploid region found in three tumors. Differences in DNA indices within one tumor characterize subclones which may arise by chromosome loss or gain.