Flow cytometric DNA analysis was carried out on 32 microsurgically removed pituitary adenomas. Additionally, the histograms of tumor cell nuclei of 7 patients were compared with those of the cultured cells from the same tumor samples. The tumors were classified into 3 groups according to the proliferation index (PI) of the flow cytometric results: 1) tumors with DNA patterns of slow proliferation (PI under 10), to which the majority of the examined pituitary adenomas belonged; 2) pituitary adenomas with diploid karyograms and PI values from 10 to 15; 3) diploid or aneuploid karyograms with PI values above 15. The third group were characterized histologically by increased chromatin content, nuclear polymorphism, mitoses, and extrapituitary infiltration of the tumor cells, and were, therefore, no longer benign. However, there was no direct relationship between the intensity of hormone secretory activity of the tumors and DNA ploidy. Cultured adenoma cells examined by flow cytometry remained stable in all cases but one.