Identifying tumor cells in body cavity fluids reliably is a well-known diagnostic problem. Since cytometric quantitation of nuclear DNA content appears to be a promising new tool in the diagnosis and prognostic evaluation of many solid human tumors, we examined its validity in detecting malignant cells in cytologically positive effusions. For this purpose, image DNA cytometric measurements, including the evaluation of DNA-ploidy and the calculation of the DNA index (DI), were performed in 80 body cavity fluids. The results were correlated with cytology, clinical course and final histological diagnoses. We used aneuploidy, as shown by interactive image DNA cytometry, as a marker for the malignancy of cells that occur in body cavity fluids with a 100% specificity and 94.8% sensitivity. Cytological investigation showed a 92.3% specificity and 95.4% sensitivity. Combining both methods raised the specificity to 100% and the sensitivity to 98.5% and had a positive predictive value of 100% and a negative predictive value of 93.8%. The DNA-index (DI) was significantly higher in malignant effusions than in benign effusions: 1.5 +/- 0.74 (mean +/- SD) versus 1.11 +/- 0.26 (p < 0.05). Along with the difficult cytological evaluation of malignant cells in body cavity fluids, image DNA cytometry can be a helpful additional method for evaluating these cells. Combining the two techniques results in a highly specific and sensitive prediction of malignant cells. We, therefore, suggest using these methods for the reliable identification of tumor cells in effusions.