The deoxyribonucleic acid (DNA) content of 10 choroid plexus tumors, including 4 malignant tumors and 3 normal choroid plexus controls, was analyzed by flow cytometry to determine whether a ploidy or proliferation rate is a better predictor of tumor behavior than histological features. Nine of 10 neoplasms had both diploid and aneuploid modal populations. One neoplasm and all three control cases had only a diploid peak. Among the tumors, the DNA indices of the aneuploid peaks ranged from 1.1 to 2.2. The percentage of aneuploid cells ranged from 7 to 99, and no distinction was made between benign and malignant. Proliferation rates were estimated from the combined S-phase fractions (SPF). The mean SPF of the control group was 0.7% +/- 0.15% SD. The mean SPF of the benign tumors (1.1 +/- 0.82% SD) was significantly different from the malignant group (7.0 +/- 1.25% SD; range, 5.3 to 8.6%) P = 0.0095. Low SPF fractions always correlated with favorable outcome. Higher proliferation rates were generally associated with an aggressive course. Evaluation of proliferation rates may help predict the behavior of choroid plexus tumors, particularly when histological features are equivocal. Measurement of DNA ploidy does not appear to have a role in the evaluation of choroid plexus tumors.