The value of DNA cytometry for predicting the malignant potential of pure dysgerminomas of the ovary was investigated. Feulgen-thionin DNA image cytometry and propidium iodide DNA flow cytometry were performed in isolated nuclei from paraffin-embedded tissue of 25 dysgerminomas. Nineteen were in clinical stage Ia1, and six were in stage III (two in IIIa and four in IIIb). Eight patients showed recurrences during a 5-year follow-up period and one patient died of the disease. The DNA index (DI; the modal DNA value compared with that of normal cells) of the tumor cells was computed from the image and flow DNA histograms. Comparable DI values, ranging from 1.29 and 3.23, were found with both types of cytometry. No correlation was found with the clinical stage or the recurrence state. Furthermore, it was striking that, although values were found strongly deviating from the normal diploid content (DI = 1.0) suggesting an unfavorable prognosis, the survival rate was relatively high. It can be concluded that prediction of the clinical course of pure dysgerminomas by DNA cytometry does not seem feasible.