Flow cytometric DNA analysis was performed on samples of paraffin-embedded and fresh adenomatous pituitary tissue removed from 16 patients with growth hormone (GH)-secreting pituitary adenomas. Histograms consistent with aneuploidy were found in 5 of 16 patients. The flow cytometric patterns on fresh and paraffin-embedded tissues yielded similar results. Ploidy did not correlate with the clinical course or the endocrine profiles in these patients. We conclude that flow cytometric DNA analysis can detect aneuploidy in adenomatous pituitary tissue, but it does not predict the clinical course of GH-secreting adenomas.