Patients with atypical adenomatous hyperplasia (AAH), run the risk of developing adenocarcinoma at a frequency of about 30%, while around 60% of patients normalise spontaneously. High dose gestagen treatment cures almost 100% of the patients, but as only 40% need this regimen, prognostic factors to identify those in need of treatment would be useful. We studied 50 patients with AAH and investigated if flow cytometric DNA analysis could serve as a discriminating factor. Number of DNA populations and S-phase fractions (SPF) were identified. Among 50 patients 3 were aneuploid, one of these developed carcinoma. Forty-seven patients were diploid, 12 with a SPF above 7.8, and 35 with lesser value. Seven and one carcinoma developed in the groups, respectively. With our DNA method it was thus possible to identify eight out of nine patients developing endometrial carcinoma. The surveillance group could thus be reduced to 30% using this method, patients in need of hormonal treatment or surgery.